First off, there are other bodies than UKBR22 that administer BR. Plenty of clubs shoot to NSRA rules which are substantially different and don't have such defined classes. There are also regional leagues such as the Midlands which allow you to have a bi-pod and a back rest, but the target card roundels are pretty tough; on the other hand they work on inward gauging which is more generous.

Back to the matter of the scope, and your budget. In my opinion you should look out for a Falcon T35 second hand, they go for about 200 quid give or take. They are a fixed mag 35x scope with a very fine cross-hair and if you get the silver version with 1/8 clicks (most of the black ones have 1/4 clicks) then you have everything you need. Take the large sidewheel off, you don't need it for BR.

I meant to add, if you want an intro to the Midlands league or the Devon league I'll forward you the contacts. You're too late for the 2017 summer comps as the entries closed last week, but it sounds like you're not ready yet. These leagues are postal, by the way; some of them you post your shot card to the scorer, and others you get a nominated person at your club to score them for you and send in the results by email.