You can't get the Evo in UK. The Evo is CDMA and the networks in the UK (and most of EU) all use GSM, so even if you got one imported it wouldn't work.
The HTC Desire HD is basically a more up-to-date, GSM, version of the Evo. It doesn't have 4G as far as I'm aware, but the UK is so shit we wont be getting 4G anytime soon anyway so it doesn't matter.