I live in Bushey, half a mile from Watford and know the area well. It is expensive because of the train route through the area, being able to get into London Euston in 20 minutes, running every thirty minutes, back and forth, from 5am to 2am comes at a premium. You are also situated right next to the M1 and the M25. Outside of rush hour I can drive to central london in approx 30 minutes aslong as there are no major traffic jams, or hop on the M1 and into the midlands in a similar time.
I used to live in north London until just under a year ago when I moved here and love it, feels like being out in the sticks for someone born and bred in London. The town centre has a large shopping centre too.
Also, Hempstead Road is the most expensive road in Watford and has lots of nice properties, many of which are worth a few times more than the £850k for this one.