Its not going to work in the UK out of the box as it is designed for 24V AC North American HVAC systems.

It seems to be able to control from Single Stage Heating up to 2-Stage Heating, Cooling and Ventilation.

I have put together a reasonably simple 24V AC Transformer and Relay fitted in a small enclosure which should allow the Nest to control common heating systems in the UK (and will hopefully offer more relays for people who have air conditioning systems and home ventilation / heat recovery systems). I hope to be offering these through my company pre-wired, labelled and with full instructions for getting it working here - just waiting on my pre-ordered Nest to arrive and make sure I get it working as expected and have everything in place to offer it here as a kit. There are still some details to work out (have contacted Nest yesterday, hopefully speak to them again today), but watch this space.