It's a bog standard key for bog standard things, it's not a security key by any means. I don't know why the railways still use it. The only benefit, I suppose, is because it's so fragile, counterfeit copies are usually crap and break if you try and use them. Not so good in respect of buggering up the lock though. Originally it was for guards, and used to lock the interconnecting doors at the end of carriages. It's use was expanded to things like SPTs and so on.
Much like the standard t key, it's not really much use for anything other than keeping stuff closed and acting as a deterrent. Millions of the things have been cut over the years and they are as common as muck. If the railway really wanted to keep things that are locked with T keys and BR1 keys secure, it'd use a decent security key. Which is why you don't find things like S&T grand masters and CDL keys floating about but you do find plenty of BR1s.
It don't believe it's really used within post privatisation era rolling stock.
There's also a driver's FG1 key which is used to activate the desk on 1st gen DMUs - largely if not totally obsolete on the national network now (I don't know whether the remaining bubble cars use them or have had them modified).
They're not exactly classified gen.