hi ceridwen-
in the uk, fathers actually have very few rights in regard to their children. if they are not married, they actually have no rights, not even visitation.
since you are married, my understanding of british law is that he will have automatic rights to the children: he will get to have them in his home for the weekend, every fortnight and half of the school holidays, unless he takes it to court.
is there a social worker involved in your case?
of course, you could breathalize him when he collects the children on friday night, and he could go home and have a few drinks immediately after. so, breathalizing will not really solve that problem, will it?
you also have an option of supervised visitation, where he can only interact with the children with a social worker present.
naive