London Transport followed from the lead given by General in standardising on the AEC Regal for its prewar single-deckers. It did not do so slavishly, as it also used side-engined single deckers (the Q class) and underfloor-engined coaches (the TF class). But its Regals were by and large long-lasting and hard-worked.
London Transport took on board at its formation a wide variety of Regals. There were the 1930 General buses, some of which stayed in business until 1953; the General "coaches" for the GreenLine semi-fast network; a few private hire coaches bought by the General; and a motley collection of buses and coaches taken over from the small operators in and around London.
Some of these were rebodied with new or second-hand Weymann bodies, for GreenLine and/or bus work. These were followed by London Transport's own designs: the mould-breaking 9T9s and the superb 10T10s - the acme of prewar design. After the war London took a few more Regals, the 14T12 Central and 15T13 Country Area buses, but these were a stop-gap before they were all swept aside by the Regal IV - the magnificent RF - in the early fifties.
Other T related sitesClassic Buses : Dick Gilbert has the original T-class page, the inspiration for my own efforts. Thanks Dick! This also leads on to Dick's site in general, devoted to half-cabs everywhere.
London Bus Preservation Trust : Cobham Museum is home to several preserved T-type buses.
London's Transport Museum : the museum includes T219 amongst its charges.