I'm not sure how roofing is done in England but here in Canada we put sheets of plywood over the rafters before installing the roofing material (tile, asphalt shingles or whatever) so the spray foam would never touch the tiles, etc. Thus, this becomes a non issue
The worst case scenario is that you need a new roof, possibly including rafters. This may not be as expensive as you think (plus you get a nice new roof).
I'm not sure how roofing is done in England but here in Canada we put sheets of plywood over the rafters before installing the roofing material (tile, asphalt shingles or whatever) so the spray foam would never touch the tiles, etc. Thus, this becomes a non issue
The worst case scenario is that you need a new roof, possibly including rafters. This may not be as expensive as you think (plus you get a nice new roof).