What do you like to do? I really liked Spain and France. I've only been to a couple cities/small towns in France and within Spain I've only been to
Barcelona, but I was a big fan of both (and I only speak a little spanish). In Barcelona I saw, of course, the Sagrada Familia Cathedral, which was mind blowing, the Olympic Park, the Picasso Museum, and
Park Guell. The best though was probably at night, at which point I wasn't really doing anything, just walking around in the lively streets, eating and observing.
France was a little tougher since I didn't speak the language but the people were nice and the weather was pretty good despite it being December. I stayed in Aix en Provence, and for me, a then-California boy, the lifestyle and city was a trip. You're in the UK though, so maybe you wouldn't be as impressed.
Really though, if I were in your position, I wouldn't know where to go either. A part of me wants to see Germany, another wants to see Switzerland, another wants to go up into those Nordic countries (as long as there's enough daylight). Well, maybe not Greenland ;)
Since you want to go 1st week of April though I'd probably stick to the southern half of Europe.
So my final picks for you:
1. Spain (weather will probably be best here)
2. Greece (decent weather, reasonable crowds, and insane amount of stuff to see)
3a. Turkey (warm and good if you're into outdoorsey stuff)
3b. France (somewhere in the southern half of the country)
If you've already been to any of those places feel free to "X" them off the list. Good luck and let me know where you pick!