Its also important to recognise that a very large portion don't want to participate - the point is they have the right to in those things that affect them.....if they want to!
I agree the UKYP suffers from inverted snobbery - although I would also say that perhaps the emphasis of their recruitment processes should be more balanced towards supporting the young people that do get selected to be able to effectively represent the young people in their areas (and not to assume that simply because they're young and in some form 'elected' they can speak on behalf of 'all young people') - we've done some work with the Cheshire Youth Parliament towards this and some info is here:
http://www.breakfastsociety.com/?s=cyp08
While the emphasis is purely on participation its always going to be open to question as to how and who participates. If instead a model of accountability was used there could be the opportunity for any young person to challenge how an organisation/service works including the opportunities and methods for young people to participate within it.
To use your example I expect a very large percentage of the 25,000 young people in your borough do not attend youth services so why would you ask them to participate? (I assume you don't therefore they can't) However if it was known that each year every young person had the opportunity to attend an event at which your service reports openly to young people about what you have been doing and how, and that during this any young person in the borough can freely ask questions of senior decision makers, they would have a much better opportunity to participate than they otherwise would. This could easily be extended to include an Annual Accounatbility Report posted on a website that allows for young people to comment - widening the opportunity to participate even further.
This doesn't detract from the value of having smaller groups of young people participate in other ways such as on committees or in projects to help with the ongoing work of the service - but it does provide everyone with an opportunity to participate which currently they don't have.
Of course the question is how comfortable are service providers with that kind of accountability? And if they're not you have to wonder why.......