1. As Benser says, there's the repulse remodel at level 66, which is actually a BB2. Mount the 15'' Mk III N guns, and you have bb3 range. (actually more range than Nelson)
2. Dunkerque has 1 R slot, but to compensate, it has 1 more support slot (8 vs. 7) compare to other pbbs. So you can level same # of sailors as in other pbbs.
3. UK has the advantage of extra support slot on most ships, so grinding is easier than other nations, but you still might want to have second BO going to Revenge line. Sure, the Revenge is pretty bad in GB, but QE, KGV, and POW are all good ships to play.