The mystery raptor from the photograph was in fact a common buzzard (buteo buteo). The common buzzard has a very varied plumage. The reason that this one was particularly white could be one of two things (I must point out that this was after substantial research :) ).
Firstly it could be simply a juvenile as they have paler plumage, it is its full size but may not have developed its adult plumage yet.
The second and in my opinion more like reason for the large amount of white on its chest could be that it is a visiting buzzard from Scandinavia, where the buzzards there are in general a lot paler in appearance due to the more snowy habitat they live in. However they cannot hunt in permanent thick snow cover and so some migrate to relatively warmer places such as here in the south of England for the winter.
Apologies for the poor quality photo, but it has been edited to make it as clear as possible :).
(Information from the BTO and RSPB).
