I don't think there's any reason that a 14-year-old can't go solo, given a mature outlook and provable skills but I'm not so sure about the whole culture surrounding soloing on the date it becomes legal.
It is highly unlikely, given the random nature of the British weather and availability of instruction and aeroplanes, that the student will reach the point that they would normally be sent solo, exactly on their birthday. Logically, one could say they're either being held back unnecessarily or being sent off too early.
It also attaches a slight air of 'failure' to those who don't make it into the air on their significant date - rubbish of course but I've overheard jokes in the bar and it could give you a bit of a complex if you were sensitive.
I'm not sure about the long-term consequences of a seriously early solo in terms of continuing in the sport - yes, it's the best time to learn and get good habits but I've seen many "solo at 14/16" articles in the local papers (good) but often fail to find these people at the club several years later (bad). Time will tell...