Anni,
I don't live a million miles from you (Selby) and tend to find it easier getting to the London Airports myself and not bothering with the connecting flight.
I usually stay over night before whether flying from Manchester or Gatwick or Heathrow. Sometimes go on the train as you can get cheap deals if booking 12 weeks in advance but more often drive and get a park/hotel deal.
Did a couple of hols from Gatwick recently and got good deals, also doing two from heathrow, Route 66 and got 4 star hotel (park in) and 16 days parking for £82. beat that for Costa Brava Radisson (4 star) + 13 days parking for £69. Usually get the good deals through my AA membership but there are other sites you can use if no a member.
Cost difference getting to Manchester or London is minimal and you save the £99 connecting flight charge.
Mick