The nicest places, closest to Hatfield, will probably cost 500K for 4 bedrooms.
Do you need primary or secondary schools? How far is your dh willing to travel? Do you want to live in a largish town or would you settle for a village near a large town?
The reason it is so expensive is because so many people commute into London - but if your dh is in Hatfield (and I'm assuming he will drive to work..) you can look at places that are north of there and maybe not on a train line which should be a bit cheaper. Give us some more info and I'm sure we'll find you something!