Possibly a slip of the keyboard, but the 21 at one time went to Farningham, Farnborough was on the 47.
and on racing days, the 21 went on to Brands Hatch.
More about the 21 at the 'London Buses Route Histories' website (and a page for almost every other London bus route, with info back to the 1930s in many cases.)
The 21 reached Farningham at weekends until 1971 (the 21A did on weekdays) until the 21A went one-man. And then in the mid 80s, a few early morning journeys on the 21 (while it was still crew operated) were started from Swanley and later West Kingsdown (there must have been a good reason for this - possibly school traffic?)
A lot of routes went quite a long way out until relatively recently -
the 47 went to Farnborough until the early 80s, and if you go further back in time, quite a few routes had extensions in to the countryside on summer Sundays, the 47 reaching Knockholt Pound until the early 50s.
Yes - the 89 didn't go east of Welling (hence not invoving Bexleyheath Garage) until 1963, and before that it had allocations (not all at the same time) from Catford, New Cross or Old Kent Road garages, so there would have been garage journeys.
New Cross continued to have an allocation until it went OPO in 1978, although latterly only on Saturdays (it was not uncommon then for suburban routes to need more buses and / or crews on Saturdays than on Monday - Friday, so a bus allocation / crew rota that did a 'city' route in the week would help out on something suburban on Saturdays, either at the same garage or sometimes a garage would have a Saturday only allocation on a route.)
New Cross took the route on fully in 1986 when Bexleyheath closed. More on the 89's history (same website)
here including a photo of an RM on a garage journey to New Cross.