Indeed so, though there are surprisingly few who fit the bill. Many hung on to a handful of front engined deckers that required conductors, even where the rest of the fleet had gone over to exact fare.
With that in mind, Ipswich Buses could well be a contender although I don't know enough about them to say when they withdrew crewed operation!
Was thinking of a small operator 100 vehicles or less. Trust me we in Scotland were glad to see the back of the early VRT’s , the Lodekkas ( even with plastic seats) were a vast improvement.
Various sources suggest August 1986, so not even close!
For information, and correcting the NXWM new story, March 1978 was the when the last ex-Birmingham City Transport service went OPO. The last WMPTE conductors finished in December 1980 (West Midlands, Malcolm Keeley, Capital Transport).
I will tell you of a couple of near misses due to the introduction of PTEs. Newcastle upon Tyne was all driver only by 1971, but was no longer an operator.
Still had AEC Renowns in the fleet in 1977, according to a picture in the Presbus Publishing 'A City of Oxford Album'.
Newcastle (as represented by Tyne & Wear PTE), may have had only front entrance buses by 1971, but certainly had two-man crews working into the 1980s (on Metro Scanias and Atlanteans).
As ever, they kept some rear-loaders, to the extent a handful passed to Tyneside PTE (as it then was) in 1973. There is a published article on general manager Norman Morton's flat fare scheme that was implemented circa 1966, possibly in quite an old edition of Classic Bus. Ultimately its abandonment led to his resignation.
I think in the case of a smallish municipal like Maidstone there was a deliberate policy, which some might consider sentimental, to keep a few older conductors on, particularly women, as they were never going to become bus drivers. Even London Country, at Godstone and a couple of other garages too, adopted a similar policy, showing a more human side than might be the case in these more corporate times.