John Knibbs or Chambers in the UK are probably better bets for BSA parts.
If the breech seal is an O-ring--sorry I don't remember exactly--you could just measure it and order up some replacements online, or even find locally. Make your best guess at the size and get a variety slightly bigger and smaller...they are so cheap, you'll probably end up paying more for postage than the rings (I've actually done that).
I am a total scope dope, someone else will have to help ya there! I'm more of a peep sight guy, though IIRC the rail spacing on BSA's is so wide that finding one to fit can be a problem.