No locked off youth gearing restriction for road racing British National Series/Championships
A year and a bit on, I'd like to answer my own question. In the UK, I've not seen anyone turned away from National Championship races for having locked off gears, and the ruling that was sent to us, wasn't necessarily implemented at the event we attended. 

However, if you are going to race in continental Europe (such as Assen Youth Tour, or any other race for that matter), then you have to ride on a specific chainring and 10 or 11 speed cassette for your category with full range, and this therefore, is not locked off. We therefore had to switch to this format for all our bikes (spare wheels as well, of course), and thus you negate the whole need to lock gears off. As I stated in my original question, the best advice is to fit the bike from the start with the appropriate cassette (you'll have to dig around to source them).