They seem to encourage irresponsible behavior, waste and littering.
I am sure a few determined people will try to put their own kits together, but I do think not having disposable ones readily available would certainly put off the majority of people who use them.
I know that "not everyone has a garden where they can do a BBQ." Thing is, though, that's actually meI live in a flat with no garden. In the country where I live, it's accepted that if you want to BBQ and don't have a garden or rooftop, you book at a leisure spot or public facility that has BBQ equipment already set up. This will be a designated area that is safe for BBQing, and there is plenty of water on tap and next to no fire risk. We have hot summers here, and certainly are very rarely getting any kind of BBQ-caused mass firesat any rate, I never heard of any.
Does the UK not have bookable BBQ facilities in BBQ-safe leisure areas?