For the bomb body, you could always use some solid Evergreen rod and sand it down to round off the nose. The tail assembly/fins is the tricky part. If the bombs have a circular tail ring, you could use some hollow evergreen tubing - if the bombs have fins, you could simply use plasticard.
I tried doing a google image search for RAF gp 250 lb pounds and came up empty although I did manage to find some shots of the 500 and 1000 pound bombs.