You can also fly City airport to Heathrow and get the undergorund but you would need to change at Acton Town and transfer to the District line which stops at Victoria, (actually there are loads of stations you could change at, Acton Town just happens to be the first).
I have flown the Belfast to Gatwick route and if I recall correctly the rail link to London is extremely regular, either once every 15 mins or once every 30, not sure which.
Guess it depends on which Belfast airport is handiest.