They have the legal power to do so if it can be operationally justified. If they were responding to an emergency that would be a proper use of their exemption. If it wasn't then theyre chumps. Personally I'd have parked on the road at the white line rather than over the bit intended for vulnerable road users.
i knew one lad, a fellow skipper, who got in a spot of bother with that. He got photographed in a bus stop on a double yellow while he bought a kebab ine evening, and the picture went round Facebook in minutes. He was offered the choice of a formal investigation, which would likely end in a written warning, or a £250 donation to the supernintendos favourite charity and an informal entry in the discipline book. He took the latter. As an aside the guy is CHIMP and should never have been put in a supervisory role and managed to get himself sacked not long after I retired, which cheered my up no end as we never got on.