It's not an issue. There's one fanatical MP who has tried repeatedly to bring in this measure for youngsters over a number of years and governments of both persuasions have blocked it on every occasion and will continue to do so.
The two reasons for that are first, that this move would discourage cycling by the young at a time when governments are anxious that the young should cycle to school etc, and take up cycling permanently. Second, cycling legislation of this kind is an EU matter which the UK government cannot enact in isolation.
The arrogance of medical people on the helmet issue always irritates me. They imagine that their medical knowledge qualifies them on issues of protective gear, which it emphatically doesn't, it's an engineering issue. We often have silly statements by surgeons like "he would have been killed if he hadn't been wearing a helmet, seatbelt etc", but they simply aren't in a position to know that, their assessment being no more than a guess at best.
We can forget this as it's destined to fail in the same way as all previous attempts by the nanny state do-gooders.
.