We did a very similar move (SE26 to Horsham) not too long ago to be with family. Slightly different perspective as one of us grew up here!
Personally, I find some of the new build areas a bit dull- they aren't particularly close to any amenities and you really need to have more than one car. We live to the west of the town centre and it's great to be within walking distance of everything we need (schools, town, shops, family etc. Not too far from station, but further than I would like...but that was the sacrifice we made for everything else being exactly what we wanted!)
A few recent posts here:
https://www.mumsnet.com/talk/property/3981750-Can-anyone-tell-me-about-Horsham
I'll repeat some of what I said on that thread (lots of school chat first)
We moved (back) here to be closer to a lot of our family, and don't regret it.
There is a lot going on, the air is fresh, lots of opportunities for our 4 year old etc. You're not far from the coast, but also have most things you need. London is an hour away, and Brighton isn't far either.
It can be a bit of a culture shock at first for those who have become used to London living (things like shops in the town centre aren't open late, and some are closed on Sundays; and a car is def needed to get to some places) but people are friendly, and there is always something happening locally (either Horsham or the surrounding areas). The area has definitely become more diverse in the past decade (a definite positive) and the has been a conscious effort to ensure the town centre remains vibrant (lots of new eateries and developments like Piries place).
The commute is a bit of a pain, though, if it's regular, and is expensive. However, it is direct and you should get a seat. We WFH primarily (one of us is in twice a week, the other once a month) so this is less of an issue for us.
ETA: Horsham also has a decent Catholic primary in St Johns (to the west of town)