It depends a bit what you mean by “good results at 7/8+”, but I have some good news for you, and some super good news.
The truth (which most schools don’t want you to know) is that outside a group of three, maybe four central London prep schools (WU, SJPS, KCSW, plus perhaps Sussex House), you can get a place at any other prep school you want, more or less any time you want (so long as you can pay the fees and are not an obvious psychopath). All other preps have spaces, and their “assessments” are accordingly extremely gentle - nothing that requires special coaching for. Any boarding prep in the country will welcome you with a rolled out red carpet, and even well known day preps which used to have waiting lists from birth etc will magically find your child/children spaces these days.
So pre-prep wise, you either want a school that will coach your kid for the 7/8+ tests for that tiny group of 3-4 London schools, which is pretty hardcore, involving lots of homework and test prep for the whole class for two years straight, starting in Y1. Not a lot of fun for kids or parents.
Or if you aren’t looking at him attending that tiny group of schools, it doesn’t really matter which pre-prep you send them to, because your son will get in to any other prep you/he fancies. Best to pick a pre-prep near to where you live, where you like the head, the vibe of the school and the other parents.
The super good news is that what applies to prep schools admissions wise also applies to pre-preps. Brexit and COVID has meant that, if you ask nicely and can pay the fees, you should be able to get your child into almost any pre-prep you want. There are a small handful of exceptions: eg some “all through schools” like Highgate, where once you are in at 3 you can stay until you are 18, but generally speaking, parents have the power right now.
You don’t say where in London you live, which is quite crucial. I would suggest getting a copy of the Good Schools Guide and seeing what catches your eye that is close to you. If you aren’t set on one of the academic hothouse preps, and you are considering boarding at some point then it’s definitely worth considering London schools which are both pre-prep and prep combined (eg Hill House, Thomas’s). That way, if your son is not ready to leave or board at 8 then he can stay on for another year or two until he is. Otherwise, you are facing a extra change of schools, which isn’t ideal.