Via the Guardian:
First, there has always been a tradition for British men originating from the Indian subcontinent to marry women from their country of origin. Families encourage their sons to do so for a host of reasons, including the cultural expectation that girls from "back home" will stay with and look after their in-laws.
The second trend is for Muslim men to marry "women of the book" (Christian or Jewish women), which is permissible in Islam. Men are more likely to work and socialise with British Christian women than their female Muslim counterparts, which leads to a higher chance of such marriages occurring.