Heart rate monitor recommendations please?

Hi i've been looking around for a while for a heart rate monitor but I don't really know what i'm looking for and there are so many around!

I want one that you can wear on your wrist only - no chest strap (if possible?), that will also count calories burned (this is the info i really need!). I'd like to find one as cheap as possible but would pay a bit more for something that works.

Has anyone got any recommendations for the above? I'm in the UK by the way.

Thanks in advance! x