I have to say that comparing the train to flying is like comparing apples with pears if people are not looking at the final destination of the pax from MME. But If your final destination is central london, you can compare and then fine the train is probably quicker - and only sometimes cheaper, but if your connecting to a flight at heathrow (even if your not through checked to your final destination) then am sorry the train isnt viable. You arrive at Kings Cross and you have to get yourself to Paddington which means travelling from North Central London to West Central London and that takes on the tube (with all your luggage) around 45 minutes (ive done it often so i do know!) and then you have to go on either the heathrow express (which costs £24 return i think) or the heathrow connect (which costs less but takes a lot longer) or you can stand (most of the time) on the tube from Kings Cross to Heathrow which must easily take the best part of an hour and is not pleasant.
Put simply if your intended destination is anywhere outside of Central London then the train is not easy - i live 25 mins west of central london and come up to Middlesbrough a fair amount during the year and 99% of the time flying up is quicker, cheaper - a lot cheaper sometimes, nicer, less hassle than by rail or by road - also everyone gets a seat on the plane which you dont by rail always!!
If an MME to LGW was started (again and i did use it a few times) - i would prefer to drive the hour to LGW, have a stressless drink in the bar and then get on a plane than for me personally travelling up to 4 hrs + on three trains and a tube to get to Middlesbrough or by driving for 4 hours each way (on a good run) for a short weekend up in Middlesbrough.
Re BMI - i think its a sad day after 40 years its come to an end - the economic climate has not helped, improvement on the railways and more people owning cars has been a factor, govt and BAA decisions have not helped but mainly BMI have killed this route by chopping and changing the times and reducing the aircraft size down to the point where people i presume cannot get on the flight on some occasions (ive travelled on the Embraers a few times since their introduction and they have nearly always been full). Ive said it before but at one stage MME-LHR was BMI's most profitable route and now its come to an end its about the future - the demand for a London route i do believe is there but by whom and into which airport remains to be seen, i agree that on a two runway LHR with only BA and BMI as domestic operators and with a packed one runway LCY with high landing charges - we can only expect something from Stansted, Luton or Gatwick.