Yes, the plane is alot more expensive if you are travelling next day but BMI and BA offer fares from £36 one-way to LHR/LGW while VLM are a little more expensive but get you to the heart of business London within 40 mins.
From Lancashire, I would definately take the plane. When travelling from my home to Liverpool Street in London, it took 2 hours 10 mins and cost me £10 return:
Travel to the airport (BLK): 15mins
Waiting in airport: 30mins
Flight: 30mins
Transfer to Liverpool street: 55mins
Simple! Saved myself £££££s by flying with FR. Obviously, this option is not currently available but, I agree, using MAN is abit more difficult and time consuming but, for transfer passengers, would they really wanty to get the train to LHR and then checkin at there or checkin only at MAN? I know which I would rather do.
Also, a late train does not mean you will be transferred onto a later flight if you miss the origional, but if your connecting flight with the same airline (or codeshare) is late and you miss the connection, you will be automatically transferred with no problems.
Its out of our hands anyhow - BMI are already keen to dump LHR-LBA to use the slots for their A330s open skies.
BMI aren't launching TATL until 2009 so don't know what you are on about! Also, LBA-LHR on sale for S08 from £37!