All depends on what your interests are and how much money you have to spend.
Also how much culture shock you think you can cope with- somewhere like Iceland, the Netherlands or Spain will be different enough from the UK to be interesting but similar enough that you're unlikely to be too shocked. Whereas you might want to dive in at the deep end with a trip to somewhere totally different. It's a sad fact that you can find English speakers pretty much everywhere nowadays so don't let language put you off too much.
I'm going to put in a vote for Slovenia- not too far, cheap to get to (this depends though what your local airport is), lots of variety, easy to get around, nice food, beautiful mountains, nice old towns, safe, efficient, friendly.
Whichever place you choose, you can't really go wrong.