For the SW Herts Consortium (here) do you mean you want a house where you would get in on distance alone, or where you’d expect your DC to sit the music/sports/academic test and get a place that way?
If you want to get in on distance, then you need to check the individual schools’ websites to see the maximum distance where people got a place - it’s usually just a few hundred meters and for some of the schools, there either aren’t many houses nearby (Parmiter’s) or the houses are in a less desirable/noisy town centre type area (Watford girls grammar in West Watford) or expensive area where you’d struggle to get 4 bed detached for under a million (Watford boys grammar).
If you are planning on getting the DC to sit the sports/music/academic test, then you should also check the consortium schools’ websites as some of them (maybe all of them?) have an “inner postcode area” and “outer postcode area” and admit more children from the inner areas, so you’ve got more chance if you live there.
You also need to be aware that the academic and music cut off scores are very different for the different schools in the consortium (I’m less familiar with the sports scores for those schools which do that test).
In general though, you could get a 4 bed detached for that price in some parts of Watford (including Leavesden), Croxley Green or Bushey. Also Kings Langley and Abbots Langley. Trains from all of those stations are pretty quick to London, apart from
Croxley which is on the Met tube line.
If you are expecting your DC to do the sports/music/academic test, unless they are a musical genius, you’d need a back up plan for a non selective school as you can’t guarantee a place of course.