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 can not 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