Formula milk manufacturers have to notify the Food Standards people of any novel ingredients. They do not have to supply the public domain (and hence their competitors) with a 'recipe' - in the same way that Mr Kipling has to list ingredients on his cake pack, but doesn't have to publish the exact way the cakes are made.
In addition, information on the packs of formula does not say where the prebiotics and other ingredients come from in any great detail - maybe there's not enough room but even on their websites, you won't learn exactly what the source of prebiotics (for example) actually is. You can hunt around elsewhere on the web and discover it. Hunker's thread shows you that the development and addition of prebiotics has been commercially-driven, on the whole.
Food Standards have to be satisfied that novel ingredients are safe (no babies are poisoned by them) and do not affect growth (they have to show that babies on formula with the novel ingredients grow at the same rate as babies on 'standard' formula). Manufacturers do not have to prove any health claims. They then permit the formula to be on sale in the UK.
However, the number of babies who are tested is small and the length of time the formula is tested is short. This is what lies behind some of the concerns about novel ingredients elsewhere - and in some countries, permission has been withheld.
Comparison between different formulas has been done elsewhere, and actually, I don't know why it has not been done in the UK. It could be done by analysing the formulas themselves, and not actually testing them long term on babies, and that would be a start. It would take up lab time, and I am not sure who would pay for it - the manufacturers wouldn't.