Hi Paul, according to Google the only standards that are legal for use on road in the UK are the current EU standard ECE 22-05, and the British Standard 6658 categorised as Type A (blue label) or Type B (green label).
If your kitemark sticker is not 6658, then it is not road legal.
It is worth noting that it is now several years since any helmets were produced with the BS sticker, so even hardly used ones are near the end of their safe shelf life, particularly those made of moulded plastic, but I doubt Mr. Plod is going to be well up on that one, it's more a question of your own safety.
As far as the police are concerned most of them are not actually the fountains of legal accuracy they would have us believe, 2 cases to prove this:
My brother-in-law was pulled on his 1978 Triumph based trike, the copper stopped him and issued a ticket for not wearing a helmet, despite being told in no uncertain terms it wasn't a requirement on a trike.
My next door neighbour was stopped for entering a chevron'ed safety box at a road junction whilst overtaking a car turning left in front of him. He pointed out that the law actually allows for this unless the lines on the chevron are continuous on your side of the road. The copper still reported him for dangerous driving, needless to say it never made court!
So, pudding basin? Your call.