Between £49 and £55 or thereabouts!
Seriously though, it depends a lot on the condition but if it's good - a collector would be very interested in that if you have all the relative paperwork - an original (genuine) BSA 'one off' would be very rare and collectable. I would imagine that normal valuations would be rather meaningless, especially if you got a couple of collectors bidding against each other? It's got to be worth a bit more than a standard offering hasn't it? It all really depends on how much someone is willing to pay?
(Why am I starting to perspire? )
Good luck with your sale!
Brexit!
ASM