Originally Posted by
Prospero
If you dissect the rule i've underlined above, you may be able to construct an argument along the lines you booked a window and an aisle seat; that as a result of the aircraft change, the aisle seat was changed to a middle seat; that you do not consider a middle seat to be a suitable alternative to an aisle seat. On that basis, you may have valid grounds for a refund but only in respect to seat 39B.
As
Prospero has pointed out so well... the above is the nub of the question. So you should definately pursue and get a refund. BA cannot get away with charging for something and then not delivering it.