How long is a piece of string?

Don't mean to sound snippy.... but you need to give more info. What's your budget? How "in-depth" to you want to see these places? (You could spend months in London and not see everything you might want to see).
What are you interested in seeing?
eg: one day is enough in Glastonbury for me, but I can't say if that's enough for you. I like Bath so I'd choose a few days there but it might bore you silly.

Airfare depends on the time of year and where you're flying from. How basic or luxurious do you like your accomodation? Do you need fine dining or are you happy with pub meals or sandwiches from the supermarket?

I've learned that people here have lots of wonderful info to share but you need to provide a starting point.....

good point taggie.

If you are looking for a inimalist stay then you could get away with 2 or 3 days in london (you'll be tored at the end and you might not get to spend time just strolling through mueseums and art galleries etc), one day in bath and one day in glastonbury.

That is if you want that sort of trip. but you need to rememebr it is "that" sort of trip and be happy about it.

yeah, bad typos sorry

8 nights in London, 4 nights in Bath, and maybe 2 nights in Swindon or somewhere near Glastonbury.

As for how much - I always tell people to spend as much as they can afford - not their life savings, but what they normally spend. I'll always stay at the top properties wherever I travel, so I'd say around 200GBP per night in London, maybe a drop to 150GBP in Bath, and a further drop to 120GBP in Swindon.

For budget per day, in London for attractions, food and drinks for two people, 200GBP per day for top restaurants with quality wines, less in Bath and Swindon.

Please come back here when you have some more specifics available to aid your decision.

That would be prices for some top-end places - you could do it quite a bit cheaper (but, of course, the less you can spend, the less you can expect).