There is a forum specifically for British parents if you have a search. And the good news is that the levels system is in many ways getting simpler from this year. At least the rules will now be standard across the country
Compulsory gymnasts (those on the elite track) do 'Voluntary levels'. These culminate in a national final.
All other gymnasts will now be doing levels from the new Classic Challenge Championship system. These are Zinc, Copper, Bronze, Silver and Gold. There is a minimum, but no maximum age for each level. Each has requirements that must be met, but the flexibility to choose which skills to do to meet those requirements.
The local Bronze, Silver and Gold competitions will also result in a regional team being selected to compete at a national final. The team is selected across all ages of gymnast competing at that level.
If you want to read more about it, then this is the BG document. Enjoy!
It's never quite that straightforward though. For gymnasts who are at the start of their competition journey, different counties and regions are still holding their own competitions for those who haven't yet reached the zinc level. They may be called different things in different places - so if you see numbers like level 7, 6 etc they are based on the old levels systems which were numbers.