15x8 +15 rims... Will they fit under an unarched MK2 2 door?

By my calculations, backspace will be about 129mm, which is very likely to touch the front struts at full lock.  The distance from the face of the hub to the outer edge of the wheel will be about 99mm, which will sit further out than the inside lip of the wheel arch, so body mods would also be required.  205s are quite skinny on a wheel of that size, so the wheel will protrude past the tyre for that English look.  225s would put the tyre edge at about the same point as the wheel itself.

15x7 ET12 is a nice fit and pretty easy to find - bought some of that size myself.  Tyre size to suit is 195/50R15.