With the way the weather has been over the last couple of years i can not see how you could say wait for warmer climate, we havent been garrenteed one a proper spell.
Would partly depend on what you are planning to do exactly.
New builds go up all year round the roofing matterials them self wouldnt be effected by rain, but there at the advantage of not fitting the insides out until the roof and windows are on. Which is where the biggest issue would arise in keeping the rain out while the roofing frames, tiling etc is carried out, yes you can normally keep most of the rain out with proper sheeting, but id reconmend budgeting for some minor water damage to upstairs rooms no matter what time of the year.
In terms of heat lose, that would be pretty minimal if you keep the insulation laid, but expect lots of wind noise in high winds.
In short it really shouldnt make much difference if you get a good experianced builder in, as the work to get the roof weather tight should take that long.
Rewired plently of loft conversions done in the winter months.