2 destinations in Europe in late May?

I'm sure that all of those cities have direct budget flights to London (check www.skyscanner.com), and of course Edinburgh has train service (5 hours). Madrid/Sevilla sounds like a great combo, especially if you have not been anywhere in Spain. Basically, I'd go for whatever is "most new" to you.

I've been to Edinburgh and, while I liked it just fine, it's not on my return list. Prague, however, is quite beautiful and I really loved it. I'm not sure how long you have to travel and what else you've seen in Europe so it's hard to comment. I can't speak to Spain- I haven't been there yet.

Prague is probably the most beautiful in May when the Petrin Hill (next to Hradcany Castle) is in bloom. There is also so called Prague Spring. If you are into classical music there is concert after concert. In May I would choose Prague, in July and August I would choose Edinburgh (not so hot), November to March I would choose Madrid/Sevilla (not so cold).

Just a heads up. We tried to coordinate a May trip to Prague last week (March, 2016) but couldn't do it. Because many Prague Spring concert tickets go on sale in December, the concerts we most wanted to hear are already sold out. Hotel prices are multiplied by factors of from 2- 5 (Yes. Five times the off-season rate.) It is possible that higher rates might also apply to restaurants, tourist sites, etc.
So we decided that we will learn from this season's mistakes and book concerts/hotels as soon as tickets go on sale next year.
Other ideas might be Budapest, Berlin and area, the Rhine or Mosel cities (just check on when RIver Cruise season begins).
I think London is always fabulous. We have been there several times in every season except summer, and we always found new things to do and see. You could always stay there the entire time and make day trips or take day tours to places like WIndsor, Glastonbury, Stonhenge, etc.
Good luck!

Travelingmom, I would be interested in if you got any decent seats this late in Hungary as well.

May is the start of "the season" so I would look for places not quite so crowded as Prague. Budapest, Sofia, maybe the High Tatras. Fortunately London is a great hub for the discount airlines and there are a lot of options. Try Google Flights. https://www.google.com/flights/#search;f=LHR,LGW,LCY,STN,LTN,QQS;t=r-Europe-0x46ed8886cfadda85%253A0x72ef99e6b3fcf079;d=2016-05-11;r=2016-05-15;tt=o;mc=e For instance fly into Sofia, drive across Bulgaria (Plovdiv, Veliko Tarnovo, Nessebar, etc) and fly out of Varna on the Black Sea Coast. Great trip. Air fare might cost $200.

Edinburgh is the easiest to combine with London, but hasn't got the excitement of your other destinations. The difficulty you will have with booking so close to your travel date is that the cheapest flights went months ago and rail fares are best booked 12 weeks out for the best rates.

30 May is a Bank Holiday in the UK, so prices will be higher around that time and availability more limited.

Madrid/Seville are interesting, but I hate eating a large meal at 11pm as the Spaniards do, which puts me off Spain. I can understand this a little more when it's hit, but I was in Spain last week, where it was 9 degrees daytime temperature and we were the only ones eating at 8.30 pm as the locals ate later.

Of your list, I would therefore go for Prague, but in practice, I would go to Berlin, which is far more interesting.

Well I live in Edinburgh and visit Prague several times a year, why not start in Edinburgh and get a direct flight to Prague (Easyjet have a few flights each week) for a few days then travel from Prague to Madrid or Seville.

Berlin is very interesting
Salzburg
Vienna
Venice
Naples
Bari
Iceland
Copenhagen
Stockholm
Vilnius

All these cities are nice to visit and easily accessible from London.

For clarity, perhaps refer to the Prague Spring Music Festival rather than just "Prague Spring", which refers to political events in 1968.