You mention Camden, so I'd hazard a guess that you're looking for schools for boys. You do not say and it may be too early to know if you're looking for highly selective places.
Senior school 13+ entrants lists will show you which preps to start looking at. You may have the choice of 11+ or 8+ entry, in the latter case you may have to register early. If your boy is academically able, switching at 11+ will be ample time to prepare for 13+ entry tests at Eton/Winchester or St Paul's/ Westminster. Early registration is also unnecessary if you're confident about the brute force of his intellect getting him through the 11+ entry exams at highly selective preps. Your local, UCS, is good, but many London day schools are logistically do-able at 11+ if it suits.
IME highly selective preps will take anyone at 11+ with just what they've spent their lives up to then absorbing. They will spot potential, welcome you and deliver the goods. Good but less academically-selective ones will also match your boy/s to suitable senior schools. If you switch at 11+ an experienced prep will do this well, so the best place to start to look is at the senior school entrants list. Senior schools may try to encourage siblings and other relatives, but mostly they go for the teachable and fee certainty, regardless of how venerable they appear. You won't be disadvantaged coming from a Camden primary if 11+ prep entry is what you settle for. Feeders are factual. If you home prep for 13+ [very rare but not unheard of] you'll be a feeder school if he gets in.
As for home support for prep entry, visiting or talking to the schools will steer you towards what you're happy to do and what they need. Camden primaries are not renown as dens of anti-learning, so I'd guess any extra support you need to give will be limited. The best guidance is the school you've set your sights on. They know best, they've seen more boys than individual parents can produce, and they know what they can/not deliver at 13+. If you choose your school well, you will probably get good news in the Spring.
Advice from other angst ridden parents, whose boys are not yours, is often irrelevant, even though it's good sport sometimes.