London Transport has a complex history that grew out of the unification and consolidation of many independent service providers. A good synposis can be found on Wikipedia.
Bus companies had heretofore been used to designing & building their own bodies, and refurbishing them, and London's Transport, in it's many named forms (originally the London General Omnibus Company or LGOC), was no exception. Even though it had created strong ties with AEC for chassis supply, it opened the Chiswick Works in 1921 for bus design and maintenance. It had the capacity to design and build its own buses (actually building chassis of its own in the 1930's) but its own manufacture could never keep pace with its fleet requirements and thus reliance upon AEC and other manufacturers increased and many & varied bus types were employed, including some with Daimler CWA6 chassis known as D-Class.
Pictured around 1954, HLW 51 (STD 122) was one of the many STD (Leyland Titan Chassis) & STL (AEC Chassis) vehicles obtained by London Transport prior to the RT. STD 122 was one of the last built in October 1946. By 1955 it was retired and hence exported to Yugoslavia as many of the similar marque were.
(Picture Graham Cann)
By the end of WW2, LT's huge fleet of ageing vehicles was straining the Chiswick Works that clearly could not cope with the fleet maintenance demand. And with the introduction of the RT in 1939, whilst things became more standardised (even though the bodies were supplied by many companies), the Chiswick Works, almost exclusively, continued to provide the fleet maintenance role.
But yet a new standard of bus was required and in the mid 50's the Routemaster was born out of work by LT, AEC & PRV (though both Leyland & Weymann built prototypes). The Routemaster would be the last bus that LT would have any design input to, other than the normal requirements of a prospective purchaser.
With the addition of the Routemaster to the fleet a new maintenance facility was required and the Aldenham Works was opened in 1956.