I'm with Yorkshireman and Lancelot on this one - the mot exemption applies to original classics, not to anything that's been substantially modified. An annual mot might be a bit of a worry, but it does provide independent reassurance that your home-built special is
safe to drive.
As for the concern that you can't drive it if it fails as your insurance would be invalid, the law says that your car has to be roadworthy if it's being driven on the road, valid mot in place or not.
If your car is declared unsafe by a police collision investigator following a serious crash you'll definitely find yourself being prosecuted and your insurance may well not pay out either.
Cellulose paint is still legal for amateur use and is really easy and (relatively) safe for the home builder to use. It also gives a more authentic period finish than modern two-pack paint systems that look too plasticky on a classic imho.
If you want your car sprayed professionally then speak to a few commercial sprayers to see if any of them can legitimately use cellulose, or try classic restoration workshops.
Anyway, welcome back and it'll be nice to see another Spyder being completed.