Thank you, everyone, for all of this advice! I should have added that we're traveling from the US (Oklahoma, to be exact). We will have one layover, then will land in London the next morning. Jet lag has never been too big of a problem. We like to do a few things the day of arrival (nothing crazy). When it all catches up to us a few days later, we just get started a bit later in the day (or leisurely stroll a museum, at our own pace). In our day-to-day lives, we're night owls, who get going around 10AM. When we travel, we're up around 7AM and in bed at 10PM.

I never considered renting a car for London. I was wondering if a car would only hinder us in Edinburgh, too. Thanks so much for the tips on that! Having been to London before, I know the tube system is very easy to navigate (especially compared to NYC's subway system), and the city is walkable between tube stops.

The reason I'm even debating this car topic so much is because so many side trips are easily accessed from London (Bath, Stratford, Oxford or Cambridge, Harry Potter Studios, etc.). When we did our four weeks in France and Italy last summer, we did extremely well using only trains. I just worry about missing some of the UK's out-of-the-way charm.

Though we live in a metro city in OK, I am perhaps too familiar with the plains of the midwest: miles upon miles of highway with far too few gas stations! I think I'm comparing the countryside in the UK to this. I'm one of those people that get a little antsy when I dip below 1/2 a tank, though!

About Rick Steves, I do own his books on several countries, but love Fodor's and DK Travel books, too. Also, I like some blogs (seat61, for example), and TripAdvisor. Rick's audio-guided tour through the Colosseum was a real let down, as was his walking tour through the Pare Lachaise cemetery. We also chose our hotel in Rome based on his rec. and were very disappointed. It was a great location, but there were much better options available. That being said, his books seem to be a good, general, jumping-off point. Guidebooks are great to read on the train.

I know that my itinerary was somewhat vague in that I don't have a "Day 1...", and perhaps I am too ambitious, but I'm still in the early planning stages. I greatly appreciate your patience!

I thought I'd be notified via email of replies so I did wait until this evening to check. (I also assumed my home country would be posted.) I was pleasantly surprised to see so many replies! Thank you all so much for your thorough responses. I'm off to research the cities and sites you all mentioned.

You do NOT want any part of a car in London >>(Bath, Stratford, Oxford or Cambridge, Harry Potter Studios, etc.)<< are all easier and faster by train.

A car is a liability IN Edinburgh but is useful in the countryside.

If this ends up a city-centric trip there is no reason at all to have a car. But if you end up out in the countryside then renting a car when you <i>leave</i> London can be beneficial.

>>I am perhaps too familiar with the plains of the midwest: miles upon miles of highway with far too few gas stations! I think I'm comparing the countryside in the UK to this.<< OMG - no! :)

First of all -- in the rural/scenic parts (most of the places tourists want to go) you will be lucky (very lucky) to average 40mph and 35 mph is more typical.

>>I thought I'd be notified via email of replies so I did wait until this evening to check.<< unfortunately that isn't a feature of Fodors . . .

Off the motorways I always plan for an average of 30mph (makes it dead easy for time calculations) and It is normally spot on.

I no longer use maps to find places in towns, GPS is much better on my phone, but for planning purposes a cheap map is vital when traveling long distances.

I once described Bath as a Traffic Jam around dirty buildings, if you want to see the clean easy to get around parts, Take the train.

Stratford Upon Avon, I like there is a bakery there with my name on it (pity its not a pub I would have had to go in) but when we do go its a case of parking the car on the hotel car park and walking.

Oxford and Cambridge carry the same dangers as Dublin only far more prolific. Kamikaze cyclists. I have an Aunt in Abingdon and a step son in Huntingdon so I know both places reasonably well but go in by bus.

And there are Motorways in the UK where you are lucky to average 30mph never mind the towns and cities.

Point being, if you want to explore by car then fair enough but most of the UK (unlike rural Ireland where I am 17 miles from a train station and 12 miles from a bus stop)is accessible using public transport.

I know this is late, but I cannot thank you all enough for your replies! I'm going to add our itinerary to this post because we had a magnificent trip. I think it might be helpful to other travelers! Side note: We had zero trouble finding petrol stations. We did get a speeding ticket in Dublin (no police officers around; we received the ticket in the mail a few weeks after returning home). We'd spent time in England prior to this, but we revisited some highlights. Five days really wasn't enough for Scotland, but it was a great overview. We thoroughly explored Edinburgh. We'd love to go back to the Highlands. If you're debating tour companies, please go with Rabbies. They are BY FAR the best, and most professional, tour company. I am definitely glad we had a car for the countryside in the Cotswolds. Also, I can't imagine being able to thoroughly explore Ireland without a car. We were very thankful for GPS!

Day 1: Fly to London
2: Arrive in London--Acclimate, hit some pubs, Trafalgar Square
3: London--London Eye, River Cruise, Globe Theatre, St. Paul's Cathedral, West End musical (Charlie & the Chocolate Factory)
4: London--Tower of London, Tower Bridge, Westminster Abbey, Royal Mews, Churchill War Rooms, West End play (Dr Faustus, starring Kit Harington!)
5: London--Portobello Road, V&A, Kensington Palace, West End musical (Mrs. Henderson Presents)
6: London--Harry Potter Studios
7: London--Highclere Castle, high tea at The Goring Hotel, Harrod's, Gray Line city tour at night
8: Cotswolds (AirBnB in Stow-on-the-Wold)--train from London to Oxford, pick up car at Kidlington airport. Blenheim Palace (3 miles from Hertz)
9: Cotswolds--Stratford-upon-Avon (loved this little town)
10: Cotswolds--Warwick Castle and explored Stow-on-the-Wold
11: Cotswolds--Oxford, Bourton-on-the-Water
12: Cotswolds--Return car. Train to Edinburgh
13: Scotland--Highlands: General Highlands Tour via Rabbies
14: Scotland--Edinburgh and Royal Yacht Brittania
15: Scotland--Edinburgh: Castle, Palace, High Street
16: Scotland--Borders Experience via Highland Express (MUCH prefer Rabbies) Melrose Abbey and Alnwick Castle
17: Scotland--Greyfriars Bobby, Elephant House, Edinburgh Zoo, musical (Mary Poppins)
18: FLY to Dublin--Drive to Limerick. Probably could have saved time flying into Shannon, but we logged some miles in the rental car. GPS made the drive easy. Rented through Enterprise at Dublin Airport.
19: Ireland--Cliffs of Moher (via boat and via coast), Burren (and Burren Perfumery), Bunratty Castle for dinner
20: Ireland--Ring of Kerry
21: Ireland--Blarney Castle, Waterford
22: Ireland--Dublin: Guinness Factory, Hop-on/Hop-off Bus, National Gallery
23: Ireland--Dublin--Trinity College, churches, sightseeing
24: Fly home

thanks for reporting back

"high tea at The Goring Hotel"
Did you actually have "high tea" or was it afternoon tea (I know how people from the U.S. like to get them mixed up).:-)

Thanks for the report back. Pleased you had a good visit.
(How did you get a speeding ticket in Dublin?)
Nice to a recommendation for Rabbies, I've been giving their link for a few years and they do Irish tours as well now.

You really managed to fit in a lot without running crazy fast. Your London days sound really busy but well planned.

The airport outside of Oxford is a great place to collect a car -- much easier than in the city centre.

You really did the whole trip as efficiently as possible. Good show! :)

(I agree 100% re Rabbies - and no, it likely wasn't 'high' tea)

16: Scotland--Borders Experience via Highland Express (MUCH prefer Rabbies) Melrose Abbey and Alnwick Castle>

can also do easily on your own - newly reopened train line Edinburgh to Galashiels (sp?) and buses - I went to Melrose, Jedburgh and Dryburgh ruined abbeys easily on a day trip.