I don't think it's entirely accurate to say that no RAF WSOs get flying instruction. Unless things have changed, RW left-hand-seat WSOs receive some basic handling training and there are about 45 minutes per quarter costed for handling training with a QHI. This often falls by the wayside but there are of course some WSOs who enjoy it and are good at it, and a fair number have crossed over to Pilot with (usually) good results.
Having said that, you can no longer become a RW WSO if you aren't one already so it might not be much help!