Nosy Be Airport
There are currently no direct flights from London to Madagascar.
To get to Madagascar from London the best thing to do is to fly on Alitalia to Milan and then on Air Madagascar to Madagascar.
Alternately, you can fly on Air France to Paris and then from there fly on Air Madagascar to Madagascar.
Click here for more information about flying from Milan and here for more information about flying from Paris.
Another option is to fly from London to Johannesburg, from where you can take South African Air Link (operated by South African Airways) on a flight from Johannesburg to Antananarivo.