Samsung have stated that countries with LTE networks will get the LTE variant, and that means the Snapdragon 800. So that's what the UK will get.
There's loads of discussion of relative performance of the two models and other differences on this forum and on other sites, so I won't repeat that.
As far as I can tell there will be only one version officially sold in the UK, whether through a network or SIM-free. In theory that should mean that a SIM-free phone will work on any 4G network, but whether it really will is the key question! Personally I am going to let other people buy them first and try it out before I buy myself. It shouldn't take too long. Also, there may be teething troubles with the O2 and Vodafone 4G networks having only recently been switched on. There were reports of some S4 LTE models sold by O2 not working on the O2 4G network at first.