heart rate monitors?

Hello! I've read quite a bit that some of you wear a heart rate monitor during exercise so you can accurately calculate how many calories you are burning while exercising. I am starting to feel like I want to get one, but some advice if you can?

1) if you have one, do you find it is a useful tool, would you recommend?
2) What brand/type do you have? the only ones I can find seem to be really expensive! can someone tell me one to get that won't break the bank? (I live in england if that makes a difference in terms of brands etc)

thanks so much in advance!

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