Estimations vary between 225,000 and 265,000. The estimation does not include the roughly 150,000 Belgian soldiers that have taken leave in Britain at one point in the war and an additional 25,000 wounded Belgian soldiers convalescing in Britain. The fullest account is given in ‘Belgian Refugee Relief in England during the Great War’ by Peter Calahan (Garland Publishing, New York and London, 1982).
Source: Belgian refugees – Wikipedia