I've never flown BA in first so this may be a silly question but how do you know that everyone else in the cabin was upgraded? Seats assigned on check-in? What about Club loads - did you visit the cabin after boarding completed, or did you check on ExpertFlyer?

BA might have been perfectly happy to sell J tickets (especially higher fare classes) with a full CW cabin and empty F, knowing that an upgrade was inevitable.

If I missed out on first choice meal, etc., as a result I'd be pretty upset but the OP paid to secure a place in the cabin - which he got. I actually think it'd be pretty disconcerting to have the entire cabin to myself.