I have a seven hour layover at London's Heathrow mid-November. I fly in at 10:00 AM on a Wednesday and leave for Spain that afternoon. This will be my first time in London and I would like to do some sightseeing rather than sit in the airport. Is it realistic to think I can catch the tube and take a quick tour on a double-decker? I think the tube trip into London takes about an hour, but what can I expect for a time frame from my gate to the tube? Will I go through customs before leaving the airport or when I arrive in Spain? How long does a tour to take? Any help or suggestions would be greatly appreciated.

Well you COULD - but certainly not by tube since it take almost an hour each way into central London - added to the long walk to/from the LHR tube station, having to go through immigration and security, etc. You would only have an hour or 2 in London.

You could catch the Paddington Express - it only takes 15 minutes each way - but with the same long walk to the station.

A more practical - though more expensive - idea would be to catch a cab over to Windsor. It is only a 10 minute drive. Spend 3 hours or so looking around the castle and walking along the Thames. then a cab back to LHR.

Meant to add - if you leave "airside" you will have to go through immigration. And on the way back you will have to go through security at least 90 minutes (2 hours is better) before your flight.

For information on Windsor, visit www.windsor.gov.uk. Here's a quote from that site telling how to get the best price for a taxi.

"The taxi rank in Windsor runs along the Castle wall. For visitors arriving at Heathrow, it may be expensive to pick up a taxi outside the Airport so pre-book a taxi for a cheaper price. Approximate price for a taxi from Heathrow to Windsor is £45.00, a pre-booked taxi will cost approximately £20.00, depending on the time of day."

It may be a pedantry but you did ask about Customs

If you leave the airport, you obviously go through immigration in London. But, assuming you've checked your bags through to Spain, they do not go through go through customs in Britain, but wait till Spain.

Arriving LHR 1000, going into towm to do some business and catching a 1700 out is a something thousands of European businessmen do every day. None of us would dream of getting back to LHR any earlier than an hour before takeoff if we'd already a boarding pass for the return flight. Mrs F still reckons 40 mins max - and never misses a flight.

Here's my suggestion:

You can't predict how long it will take to get through immigration, so have a "short time" (like Windsor) and "more time" (like a bus tour) option planned.

Take the Heathrow Express (15 mins journey, every 15 mins) to Paddington. Both The Original Tour and the Big Bus Co (the two major hop-on, hop off operations) have stops on Praed St close to the station, though you may need to get tickets (eg via the web) beforehand. Allow roughly 3 hours.

If it's a really miserable day, and sitting in traffic looking at rain-sodden buildings doesn't appeal, try getting the tube to South Kensington (40 mins from Heathrow)then losing yourself in your choice from thre V+A, the Science Museum or the Natural History Museum. Or if you're a masochist, take the same tube to Knightsbridge and judge whether Harrods really is the most unpleasant shop in the world (then go and look at imaginative clothing at Harvey Nichs a few doors down)

flanneruk's suggestion to have both option in mind lets you go with the flow. One thing - it is definitely NOT necessary to buy tickets for the sightseeing bus tour ahead of time. If it is a staffed bus stop you buy your ticket on the pavement (sidewalk) from the agent. If it is an un-staffed bus stop, you buy it from the driver. Having £ cash helps a lot.

So since you don't need advance tickets for anything (Paddington Express, bus tour, taxi to Windsor, Windsor Castle, etc.) Just decide after you get to LHR. In fact you may be so jet lagged and tired you won't want to go anywhere.

I'm no help with London, but as far as general airport procedures & layover time...

Is the 7 hours your actual time between flights? If so, you do need to subtract ~1 hour to get yourself off an international flight and out of the airport, then 1-2 hours back into the airport and onto the next flight. My point is only possibly your time is more like 4 hours to spend, rather than 7.

If I'm booked entirely on a oneworld alliance ticket (AA from JFK-LHR, then on Iberia from LHR-MAD-JNB), do I need to worry about customs or immigration when I arrive in London? I'm not planning to leave the airport, but it's been so long since I had an international connection that I can't remember the policy.