Yeah, if he's not baking it AND you've got typical (cooler) UK weather, a year sounds even a little on the short side. It depends on resins used, amount of catalyst used, whether or not he used any "fixers" (used to speed up the curing process) and so forth. If you could "bag it" in a little room fabricated of plastic sheet and could elevate the temp in there to 30 degrees C for a few days to a week, it'll speed up the curing/outgassing of the resins, but, to be honest, if it'll take you most of a year to get the kit built before you go for paint I would probably just go ahead and get to the paint whenever - it'll probably be fine by the time you get there.
Sounds like it would be a good idea to find other buyers of that body on some UK kit car web sites and see what other people had to go through and use that as a guide. In the meantime, you can go ahead with the build after a couple of months out of the mold. Just wait for over a year before you start thinking about paint prep.
Just my two pennies....
Oh, and if you can post a picture of your sagging sill (or send it in an email) we might be able to give some better advice about straightening it. Fiberglass usually doesn't take a bend - in other words, if it's molded curved and you later straighten it, it'll go right back once the straightening device is removed.
If your builder is recommending that you put something under the sill permanently, that's another matter. The CMC kits, that a number of us have, run a body sub-frame made of 2" X 4" boxed steel right under the sills. If that wasn't there, those sills wouldn't have any strength at all and would be all over the place. The boxed frame sits on top of the pan edge.
I have a CMC body sitting on the floor of my shop, and I can lift it up with a winch in the ceiling. If you want, I could get it up and take some pictures of what's under the sills for you, or anywhere else for that matter. Just ask.
gn