Hi OP, I’m currently looking at houses in Billericay, Brentwood area and Chelmsford at around your budget.
I really like Billericay, it’s a pretty town, mix of height street names and independent shops. It has a Waitrose. Lovely parks and countryside. All the primary schools are good. The 2 secondary schools are good, not sure if Mayflower still has an outstanding Ofsted. Mayflower has the better reputation out of the 2 schools which means the house prices in that catchment are higher than the sunnymede area. It’s close to Chelmsford and Basildon for cinemas / leisure activities etc.
Chelmsford - as a PP mentioned, most schools in Chelmsford are good. Avoid the Melbourne area and Hylands secondary school catchment. Springfield, Chelmer Village and Moulsham have good schools (avoid Springfield primary but plenty of others).
Aside from the grammars, Boswell and Moulsham high schools are good. The all through school is in the new development - Beaulieu Park. It looks nice but I’ve a lot of the houses in my price range have completely open plan downstairs which you may or not like. Also tends to be 1 allocated parking space. Chelmer Village is also like that for parking. They’re building a new station new train station (near New Hall school) due to open in 2025. Great Baddow is also very nice and some of Galleywood but further from the station.
Chelmsford has a lot to do and very regular train to London.
Personally I’d avoid Wickford. Rayleigh had a nice reputation for families.
Shenfield / Brentwood / Hutton / Warley all mostly nice but more expensive. Especially Shenfield which has the fast train. Travel into London is a bit cheaper, Shenfield is the last station where you can use oyster / contactless.
HTH.