I think that it's not a nightmare precisely because it is so difficult to find a compliant rear light. AFAIK nobody makes rear cycle lights in this country, & overseas manufacturers ignore British standards. If a law becomes impossible to obey, one cannot be penalised for not obeying it. Well, as long as one shows willingness to obey as far as is possible, e.g. by using a good, visible, rear light.
I should have kept an old BS6102 light so that if ever argued with in court I could show it lit up beside a new non-compliant light, remind the court what the purpose of a bike rear light is (& why there's a BS for them), & ask which light fits the purpose better.
The Cateye TL-LD570 might, perhaps, from the wording on the Cateye UK website, but it doesn't quite say so explicitly, & that's the only one I've found.