Geez I'm not much of an expert regarding UK, but you can't do much wrong with London and Edinburgh (even if I haven't been to Scotland myself).
York is pretty neat as well.
Have only been to London for a day, so I only did the standard tourist program. Take a look at Tripadvisor for some popular sightseeing stuff.
During my Iberian Peninsula trip this year I slept in various hostels and was very satisfied. As long as the whole point of your trip isn't to calm down, but to see new things in various cities (which is obviously a cause for some stress) it's definitely good enough.
Aside from RyanAir, EasyJet is also very popular in the UK.
Personally, I buy everything I can beforehand, so I don't have too much hassle with money or printing tickets and whatever else could go wrong. I'm also pretty stingy when it comes to food. Most of the time I only go to local supermarkets and buy baked goods. Something that is cheap, fills me but is also easy to transport.
EDITED my post.
