In Harlow we've got some roads unrestricted (ie 60), some 50, some 40, some 30 and some 20. It gets confusing - you get essentially identical roads with different speed limits.

Twenty as a maximum for any road in a residential area (it with houses facing on to it) makes sense. Mind, there are plenty of roads where parked cars can mean that it'd be sensible to keep well below that maximum.