Route 631 is unique; the only Transport for London school service operated by midibus. One bus provides four round trips in each school peak over a circular route between Golders Green Station and Henrietta Barnett School in Hampstead Garden Suburb.
631 is a recent renumbering; previously the route was numbered H1, a variant of the Hampstead Garden Suburb route H2 (which was itself numbered when H1 was in use in the Harrow area). During the off-peak the bus operates route H3, an hourly service which largely operates along roads in Hampstead Garden Suburb not served by other bus routes.
The H2 started life as the Hampstead Garden Suburb Dial-a-Ride, which commenced on 19th October 1974. Intending users could phone a central control point (located, I believe, at Golders Green Station) and have one of the 16-seater FS-class Strachan-bodied Ford minibuses pick them up outside their front door. There were also fixed stops at Golders Green Station and at Market Place in Hampstead Garden Suburb, and passengers boarding at these points simply told the driver where they wanted to get off. Although the service was successful and proved the need for a bus service within HGS, it was expensive to operate. From the end of February 1976 it was replaced by the H2, using the same minibuses but operating to a fixed route and timetable, with hail-and-ride facilities over most of its length.