If we are talking about pure domestic travel and not connecting to another flight there is no contest if one starts or finishes ones journey in London and travelling to the North West (I assume the North East is similar) – the train is much faster, the overall journey more comfortable, much better service and a regular frequent service. BA to or from LHR cannot compete. If your journey starts or finishes on the M4 corridor or such then some elements equate and in some cases using LHR could be quicker. You choose depending on actual destination or departure point ie total journey time v cost with ancillary issues thrown in – it is usually an easy decision. Living in the North one would never fly into LHR to get the tube into central London – those days are long gone which is presumably why the MAN/LHR route is predominantly connecting traffic. Scotland is of course a completely different situation.