Not exactly a 'custom' as such. The vast majority are given to the hotels (and also hospitals, care homes, prisons etc, etc) by an evangelical Christian organization called the Gideons and have been since long before Britain became 'multicultural' in the way it is today. They give a New Testament to each school-child as well, or at least they did in my day; I suspect that might be different now. In a 'Christian' country that's fair enough, IMHO. That said, if people are offended I'm sure the hotel staff would gladly remove the Bible from their room for the duration of their stay if asked. Many London hotels now also have a copy of the Qur'an in each room as well.