The two main players are EuroUSC and the Resource Group. There are, I think, 2 others. They are CAA approved organisations, and if you go through the their training program, which mvigs costs above, you are well prepared to submit an acceptable SRG 1320. ie the CAA is much less likely to find fault with your operations manual and your working protocols. This is worth the cost as the CAA is very pedantic and moves with the speed of a geriatric slug. So any fault with your submission will take some time to resolve.
I too went down the EuroUSC route.