About 4-5% of the UK's population are Muslims, 2% are Jews and 1.5% are Hindus which means they are forbidden to eat beef-based gelatine. The problem is that food declarations do not have to specify from which animal(s) the gelatine was sourced, and many manufacturers don't bother with halal/kosher certifications due to the extra work involved (the certifications usually have to meet strict ultra-orthodox standards, whereas many people don't care about the specifics but only about the kind of animal).
As for the number of vegetarians and vegans, the UK reports 11% who live at least vegetarian or stricter per [1].
Unfortunately I haven't been able to find vegetarianism and religion correlations for the UK, but in any case it's safe to assume that anything between 11 and 19% of the UK's population are in one way or another following ethics that refuse gelatine-based products. That is not "nobody" at all.
[1] https://www.food.gov.uk/sites/default/files/media/document/f...