If I were in the UK, and wouldn't want any hassle, I would go with Dianemo. They also have a software only package.
If I were in the UK, I would only go with Z-Wave, if I do not have an option to go with KNX. Otherwise I would choose KNX.
Having said those things, I am sure others will be able to provide more indepth Z-Wave knowledge.
Looks like there is an option for KNX in the UK: http://myknxstore.co.uk/ ...but without setting up an account I can't see the prices.
Anyone able to advise if KNX is better value than ZWave, or easier to control, offers more features as a protocol, or perhaps there are more device options, compared to ZWave? My impression when looking at ZWave products was that they were expensive, especially from a the view of savings in energy bills from investment in them.