Not sure about the aircraft question sorry, although I doubt that they would downgrade to a smaller plane as I think that it is a really popular long haul route for BMI and is a code share with LH and UA.
I've flown PE a few times with BMI, and have flown WT+ once with BA.
With BMI the seats are the same (whereas WT+ and Virgin both have wider seats) but with the extra 6" of legroom which makes a massive difference if you are a taller traveller.
I've always got good service with BMI (much better than when I flew WT+), the crew seem very attentive as it is only a relatively small cabin and you get a couple of extra features on normal economy such as priority boarding and checkin, juice/water when you board and a power point for laptops.
I don't think that it deserves a massive premium on the normal economy ticket price, but if you can get it for not much more (or free if you are BD * Gold) then it is definitely worth it.