Thank you @Malmontar for your response, I appreciate you taking the time. Absolutely, it's all about the overall ethos. A relaxed uniform policy usually suggests a less conservative school and rules and regulations simply for their own sake do not bring out the best in our son, as was our experience at primary school.
Our son is currently thriving academically and creatively in a London school and the Headmaster has been lenient towards his coloured hair to date, justifying this by his impressive performance, maturity and kind consideration towards his peers and teachers. Moving forward, the hair colour is being discouraged as it is not within the school policy, although exceptions can be made - very soon he will be travelling abroad for a school music competition and it has been acknowledged that image and 'unnatural' hair is part of the package in rock and roll! This is an example of the type of open attitude we believe is essential in a school.
His hair is important as he leads a punk/new wave band and they have gigs lined up this summer. Last year he would perform/busk in various London locations and the recognition he would later get surprised us all. We must have been stopped randomly half a dozen or so times from people in the street - the reality is that his hair made him instantly recognisable. One such encounter resulted in a summer party gig on Eel Pie Island and I'm sure it cannot be denied how positive this is for the confidence of a child, let alone how valuable the experience is for a (potentially) budding musician.
Hair colour also allowed him the opportunity to present himself as he saw himself - a young creative who is challenging social norms and thinking deeply about the world and his place in it. The songs he currently writes and performs reflect this. Surely no bad thing for a child to open their minds and explore possibilty in global uncertainty. And his hair certainly brings no harm!
Mumsnet has served as a great starting point for possible options and it is wonderful to know that there are schools which are more left than right in their approach. His current school is a prep school and we may need to explore other options down the line, hence my debut question. The outrage from some users - whilst quite funny - may make it my first and last. I think 'sisterhood' and enlightened conversation can overall be better found elsewhere. I am, however, grateful to those who have helped with suggestions and send my sincere thanks to you.
Over and out...