you're right - other OneWorld airlines do operate "First" lounges which are also top-tier status lounges for those flying lower classes - just at LHR (Terminal 5 in particular) the volume of passengers does crowd the lounges quite badly at peak times - seems worse than some equivalent OneWorld lounges (i.e. Cathay in Hong Kong, but maybe that's just me or something I observe at LHR Terminal 5 is where I end up most frequently on my own travels...)
the point I was really trying to make is that BA "First" in Terminal 3 at LHR is not equivalent to Concorde Room in Terminal 5 - whilst it has the same access rules, Cathay First at Terminal 3 arguably comes a lot closer partly by reason of being quieter (e.g. because many BA and other OneWorld status flyers through Terminal 3 who would actually have access don't know about it....keep it to yourself or everyone will desert the BA and AA lounges!!)