Visas are required for all those entering Madagascar from abroad with foreign passports. The process for getting a Visa is very easy and usually completed within 24 hours, provided the conditions are met. For example, all passports must be valid for at least six months after return date, and you will be required to submit photographs and a handling charge. An application form for applying for a Visa to Madagascar can be found here and given to the Malagasy Consulate in London. Alternatively, it is also possible to get a Visa at the airport on arrival, though for peace of mind it is easier picking one up in advance.
The address for the Malagasy Consulate in UK is:
16 Lanark Mansions
Tel: +(44) 208 746 0133
Fax: +(44) 208 746 0134