Last year I started the 'Puffer in a Wool LS Endura base layer, a Stealth jacket, (with all the vents un-done), fleecy Trek tights, Endura super light water proof shorts, wool socks and Northwave winter boots. As it got colder I did up the zips on the jacket. That was fine for the first 10 or so laps. I put a Patagonia R1 midlayer and Rapha winter hat on in the night. It all worked perfectly. Next time I'd take more sets of gloves though, changing them every couple of laps would be better than frost-nip.
Stealth tights are really nice. I would go for the lighter ones, and if you are really worried a second set of thermal tights underneath. Thats good to lower than -20 and a much more flexable set-up.
The Stealth stuff is pretty much ideal for UK use. Proper waterproof, breathable, and very comfortable.