It's fairly simple really. If you're arriving in CW or F on a BA longhaul into London (any airport), you have access.
If you're Gold and you're arriving BA longhaul into London in any class, you have access.
No guests.
There is no access otherwise - even if you are in CE inbound and F outbound from LHR, they will not make an exception. Trust me, I have tried.

For reference, it is summed up here, in the first post: http://www.flyertalk.com/forum/briti...-database.html
and here: http://www.britishairways.com/travel...s/public/en_us