"In Germany they have a system where the primary school suggest which school is best for each pupil, I'm not really sure how this works in detail though but it might be better than a single exam."
That sounds like a recipe for prejudice, families following patterns, etc etc.
The whole point is that in a comprehensive system there is no NEED for these intricate contortions to get around the various pitfalls and unfairnesses of selection. Late developer? Gets into top sets in Yr 8. Tutored and hot-housed at 10? Levels out by Yr 19, and doesn't occupy a place by a late developer acing up in the inside.
My DC's comp doesn't have overall high averages (except its FSM %) but the top sets do excellently, and the Inclusion classes get a really good education and fulfil their potential.
My Dc with a string of A and A at GCSE was not 'brought down' by the middle attainers and slower learners whose (good) education and results bring down the overall A-C average.
And we are not in a posh area, not by a long chalk.