Depends on how old you are and what you like. Do you like food ? London has the best curry in the world. Do you want nightlife then its London again. Like Architecture then visit Oxford or Cambridge they also have some amazing restaurants.
You can do a night time boat trip on the River Thames last 2 hours and you get a great dinner on most of the boats. Shopping in Oxford Street or Bond Street is an experience if you have large amounts of cash to spend. Or visit Carnaby Street, changed a lot from the 60's but still an interesting place.
Like watching people, then Piccadilly Circus is a good night time hangout. Used to be some great clubs nearby but that was 15 years ago before I left for San Diego.
