Firstly, I second the comment about Gatwick. Expensive doesn't quite do it justice!
If you're flying commercial to London then Gatwick and Heathrow are your main routes - but both are prohibitivly expensive for the usual GA / Light twin budget. For South London, get the ferry pilot to drop it at Biggin Hill - for North London, Denham or Elstree would probably be a better option. Biggin will still have a handling and parking surcharge so if your wanting to leave it for any length of time before you pick it up then Denham, Elstree or Redhill would probably give better parking rates.
If you are tanking, then (obviously) remember your 337 - your ferry pilot will be able to determine the best route over the Ocean, but I think that Goose / Greenland / Iceland / Scotland is a better, safer option. Where you have the tank removed on this side of the Atlantic may also enter into the routing equation.
During winter months, the option of routing Norway/Sweden as previously suggested may give you longer delays with the weather. Your choice of routing through France down to Cannes the across Northern Italy VFR would be one way to go with the warm weather. If you are venturing anywhere near Italy, make sure that wherever you route - double check then tripple check the quadrouple check that they have Avgas available, then plan your route carefully so that you can you always get out of wherever promised you they had it then wasn't able to give you any when you arrived. Lots of places in Europe are 'cash' only - so make sure you have a fair supply of the folding stuff!
If you are IFR then there are several airways routes available.
If I can help or advise any further, then PM me.
DD