Polar is the leading line of heart rate monitors - their cheapest one with a calorie counter is probably around the £40 - £50 mark. (Used an online converter, I think that's accurate.)
Timex is also a fairly good brand, and they are usually more affordable.
An important thing to consider with HRMs is to make sure that the one you buy comes with a comfortable strap. Trust me, it's worth spending a little bit extra to get something that won't get irritating in the bra-strap region after 45 sweaty minutes on a treadmill.
All the best.