If you're thinking 1991-ish, then Maidstone & District still got as far west as Gravesend on the routes from the Medway towns and so on. Their Gravesend depot closed in the mid 70s and the town routes passed to London Country (Gravesend was one of those towns where - broadly - buses to each side of town were different operators)
M&D historically had a residual few journeys on a Dartford - Sheerness / Leysdown service, and some did later extent as far back as Thamesmead - I've seen photos of double deckers operating this service, but not sure it ran as late as 1991.
As others have mentioned, there was the Invictaway on Medway towns - London express services - I can't remember ever seeing a standard bus running as a duplicate / replacement.
There was a brief flurry of competition in Gravesend following deregulation, but don't think M&D took London Country on (M&D were more occupied in Maidstone.) It's possible one of the Gravesend independents had a VR or two, but can't immediately think of one.
There was also some competition in the Dartford area from Transcity - who put green buses out with the 4xx route numbers that had been established in the 1930s in competition with Kentish Bus who made a complete break from the past with yellowish buses and a new 1+ route number sequence. I don't think Transcity had any VRs (can't remember any double deckers at all)
Eastern National historically had a service from Southend through the Dartford Tunnel which would at one time have had VRs and at one stage this ran through to central London via Blackheath etc, but I think it was mainly Olympian coaches by then. And Southend Transport may have been running it as a through service to Brighton by 1991, or it may have ceased altogether.
If you're talking about inside the Greater London boundary (i.e. anything within zone 5 / 6) then deregulation (outside London) led to less through services across the boundary, and LRT (as then was) taking many of the routes over.
Historically, London Transport's 'red bus' area pretty much stopped at Erith / Bexleyheath with only the former tram / trolleybus route 96 venturing further east.
Routes 401 and 492 were London Country routes until 1988, and some of the other current TfL routes around Bexleyheath were LC rather than LT routes until then. The 428 to Erith was part of London Country / Kentish Bus 480 until the late 90s. From memory, the 480 went over to low floor Wright bodied Scanias somewhere around 1990 - those buses stayed on the route until quite recently by which time they were getting somewhat tired for a trunk route.
1988 brought the 'Bexleybus' scheme and its blue / cream livery to Bexleyheath and surrounding areas - also Boro' Line Maidstone (who set up a base at Crayford) and Kentish Bus running some routes on tender. M&D never got in to the London market.
Bexleybus used ageing DMSs (that had already been sold off by London back to London service and were being replaced by Routemasters in Scotland) as well as a few new Olympians and LT Leyland Nationals. And minibuses, of course.
Boro Line had quite an array of hired double deckers (I remember Ipswich, Nottingham, Hull buses at least - all Atlanteans) until newer buses arrived. Kentish bus used a mix of Nationals and Atlanteans (including some Alexander bodied examples ex Glasgow.)
Bexleybus did gradually revert back to standard red bus livery rather than its blue / cream. I can't remember exactly when, though.
Some red buses (e.g. routes from Plumstead and Bromley garages) continued to reach Bexleyheath through this time.
VRs did get on to (what was then LRT) tendered services in NE London under Eastern National's Citybus operation, but can't think of any VRs that ran south of the river.
