I don't think Martinair's operation caused too much of a flutter, given that it only operated for 1 winter only. THe consternation was caused around 2001 when VS pulled their own aircraft out of MAN and replaced it with an Air Atlanta 747-300 without telling travel agents what they were doing; the travel agents were trying to flog amenties on board an aircraft which was never destined to operate on the route for the whole of the peak summer period.
As for VS 787s, I can't see them effectively operating stretched out W patterns out of LGW as they currently do with the 747; the news that they are seeking interim lift of 12 aircraft also came again with the notion that they are intending to operate them out of LHR, LGW and MAN. LGW may have some purely on the "thinner" leisure routes with the 744s (and their replacements) operating the bulk of the LGW services, A340-300 routes out of LHR will probably go the way of the 787. MAN is more suited for having 787s based operating the pulled BD leisure routes and affording a greater degree of profitabilty to MCO (with perhaps greater frequency?). I would like to think VS would also consider the prospects of non-bucket and spade routes as I'm sure if they offer the right config (unlike certain airlines), they'd get themselves onto a winner.