Oh dear, this is very much what I didn't want to do here but, no, per WP:Engvar it should be nothing of the kind unless you can show me where WP:Engvar says "always use 'an' in BrE in these cases". It's a form of synthesis to say 1. ENgvar says use BrE here so 2. this should say "an historic", because it requires me to accept the unspoken step 1a. the correct BrE usage is "an historic". This I do not accept: I know it is used by some BrE speakers but others, including me, find it archaic and frankly embarrassing. So I see no justification for changing it, and I feel it is important to assert that people who want to say "an historic" cannot simply hide it behind EngVar and BrE. But this is what I mean about trying to avoid the I know I am right about this bit because of course we both know that we are and we can both - well, if we could be bothered - go on asserting that I am right and you are wrong ad infinitum. I was hoping for something a bit more, er, policyish than this. Best wishes