Well, it says nothing about difficulty in securing said location, nothing about impossibilities, so my pick is.....

Probably one of the most haunted locations in London. I've sensed some ghosts here, one of them on a rather twisty set of stone stairs and realized that dizzy spells on those stairs is not a good thing. The GAC did go here while they were in England, but it was during the day. I think a lockdown here would be WICKED.