There are lots of things that have to be considered here though

Just considering the adults who do well but who were not top juniors (as opposed to top juniors who do not become top adults, as that is facile, it's just maths):
For instance, countries with a strong domestic scene will have some juniors who play far less ITF events because they already have good tennis to test themselves against

Equally, players from far-flung countries or where travel was tricky or expensive will probably play less ITF events

Equally players from countries that do not support their players in any way will often play far less juniors - there's
quite a few Eastern European girls who have been clear that they've stopped juniors before they wanted to, effectively, because they need to earn money and so need to move to adults, and no one cares about unpaid juniors (the Fruh. sisters said something along those lines pretty recently)

So, a lot of those players will look like they weren't very good juniors although, in fact, they were, just not fully engaged int he ITF system

Which backs up that it doesn't matter how you get there, which is definitely true

But practically none of those factors apply to the UK so you'd expect the UK players to be more 'mainstream' in their junior achievements - which still doesn't mean they have to be top-50 or whatever - and there's always room for a few JoMos - but the majority will be more 'normal'