I would agree with SimoneJM and say that if you're including airfare in your budget, it's going to be tough to last a full two weeks in the UK with $2500. That's not to say it's not possible, but it will require a fair amount of advanced planning and careful budgeting.

Croatia, especially Dubrovnik, is not as cheap as it once was, but there is a lot more to work with on the budget end of things. As others have mentioned, May should be early enough to miss the crowded high season and enjoy things on your own pace.