Longhaul - LHR or MAN
There's also the cost issue as well. There is often a premium (call it something else if you like) when travelling from a regional airport on a longhaul flight. For example, when travelling to Singapore last year with SQ I found MAN fares to be approximately £100 more than the equivilent flights from LHR. As I'm between the two, the choice of LHR was quite simple really. That's aside from the fact that you're more restricted for options (airlines/routes/number of flights per day) when flying from somewhere that isn't LHR.