Hi there - I live in St Ippolyts (on the Gosmore side - the village is kind of split in two by the Hitchin road)
THere are loads of trains into London from Hitchin, the fast ones are under half an hour into King's Cross. I cycle there from St Ipps and it takes me about 10 minutes. DH drives as he drops the kids off and I think it takes him a bit longer. With no traffic, it's a 5 minute drive into town.
Primary schools - the way it works in North Herts, you get priority for your nearest secular school. So we essentially have a choice between St Ippolyts and Highbury Infants in Hitchin. I have looked around both in the last week (applying for a school place this year) and they are both good schools. St Ippolyts has the edge for me in lots of ways though, enough to get past the 'church' side of things (we are atheists but from Jewish background, so not C of E in any way). The very first thing the headteacher said was to ensure us it was "a village school first, then a church school". I can give you more details about this by PM if you like, but essentially, it operates a catchment area, which has been widened recently, before any criteria about church going.
Housing - very little in that price range comes up in the villages. We have a large three bed with plenty of garden but it cost us £440k a year ago, so think it is worth more now. Also, it was a total 70s museum and we have had to completely modernise, including rewiring, re-plumbing, re-plastering, new bathrooms and kitchen, the works. So you may end up with a bit of a project. Saying that, there are smaller three beds, including older houses with downstairs bathroom etc, and there are some 60s built houses over on the East side of St Ipps in a really nice location near a park and backing onto woods.
Other villages worth a look would be Ickleford - has a good primary, don't know if it's church; Willian and Wymondley - although very few amenties like shop, pub etc in Wymondley. Willian, I would check where you might end up for secondary school - these are better in Hitchin than in Letchworth. Codicote is also very nice, but I think your commute would be from Welwyn there. Baldock is a small town, but might be worth a look. Don't know if there are lots of big gardens there.
If you do move to St Ippolyts/Gosmore and get some chickens, come and say hello - I am thinking of getting some when life is a bit less crazy (currently still renovating while juggling a part-time job and two under-4s!)