On the 15.40 (IIRC) from MAN so I suppose I shall have to queue up with the rest.I had thought of collecting my bags at LHR and checking them in again at the FIRST check-in at T4 for the experience but cannot be bothered TBH. Seems a shame that F connecting PAX don't get any special treatment at MAN though.
Yes you'll have to queue most likely, but it is never too bad in the early afternoon.
The baggage problems at LHR seem generally sorted now so I would certainly check my bag through, and indeed did so a few weeks ago on just such a connection with no hassle. FIRST checkin at LHR is really nothing special, you just don't have to queue quite so long, and there is a bit more space between the desks.
You do get to use the lounge at MAN which is one of the nicer Terraces around, aside from that there really isn't that much extra special treatment you could expect.
Enjoy BA FIRST! It is still really one of the nicer ways of travelling these days!