...or get them flying on anyone else but BA/BMI, and there are lots of other options ex MAN - AF/KL/LH/QR/EK/SQ/AA/DL/TK to name a few who can provide as many connections as LHR.
Not may northern folk are that loyal to BA, and wont mind connecting elsewhere should it be more convienient than the train / car to London to get on the UK carriers...so maybe not so many air passengers would be actually be lost, and the carriers that remain at MAN would gain reward in extra revenue by filling all their seats, at BA/BMI's expense. As someone said in a previous post that would be a serious body blow to BMI, but more so BA.