So in the UK there are different things that happen depending on your age for the majority of crimes.
In Scotland:
0-6 - Nothing, not criminally liable.
6-16 - Gets sent to the children's panel and they'll normally waive it off.
16+ - Adult court.
In Scotland they're an adult at 16.
In England:
0-10 Nothign, notcriminally liable.
10-18 - Youth Court for minor stuff, crown court for major stuff.
In England they're an adult at 18 but they still end up in the normal system when they're above 16 as far as I understand. (I'm Scottish, so most people I knew who went through that system did it in Scotland)
And in the UK there are time bars, so if you get arrested and charged they have to have you in court in a certain amount of time. Unlike in the US system, the game of delaying going to court doesn't work or even really allowed. The court date is set and that is when it goes to court. And if you were arrested for something at a certain age then you would be convicted based on that as far as I know. However, if you get arrested at 20 and convicted once you're 21 you go from a YOI to a HMP. There is no real difference other than the HMP is more relaxed as older convicts don't feel the need to prove themselves daily.
I am not sure what you're referring to but the strategy of delaying going to court is a may strategy used in US Courts. The state may have to be able to go to court within a speedy time but it doesn't stop the defendant delaying it.
Secondly, no the US protections are not stronger than the UK. The UK protections are that strong that if they don't get you to court in a certain amount of time they can't convict you. It's 6 months if you're held in custody and longer if you're on bail. It's quite common from my understand via documentaries of prison/jails that people spend a year in jail waiting for their trial and it's possible for multiple years if bailed.
Or do you get tried as a minor but sentenced as an adult?