One thing I wanted to establish was whether these were covers produced in the BFPS series and stocked by the BFPS. I'll check in their catalogue, although that only tells what they last had for sale, rather than whether they produced these. (The catalogue can be downloaded from http://web.archive.org/web/200906290429 ... 270209.pdf
These are older covers. Buckingham Covers
deal with NAM now, although they may not have when yours was produced. They look like Connoisseur Covers, and another company which I can't now remember.
Westminster Collection have done one-offs for RBL, but again, not this one.
Most are fairly common although signed ones are always better than unsigned. Buckingham Covers deal in signed covers.
Philatelics GB stamps info: http://www.norphil.co.uk
- also /catalog for our ecommerce site, blog.norphil.co.uk, shop.norphil.co.uk and Ian_norvic on twitter