Brighton & Hove are now, I think, fully low floor for the regular stage carriage routes, having fairly recently disposed of their last batch of step entrance Scania/East Lancs double deckers. A few step entrance Darts remain as trainers. First have begun scrapping some of their ex-London Marshall-bodied Dart SLFs, due to the chassis rot that most Darts seem to end up suffering after ten years or so. First have a policy of endlessly 'recycling' their ancient vehicles and running them until they drop, usually looking a right state by then. Quite handy if you want a bit of 'stepper' bashing though!
Many of the smaller independants seem to be getting their hands on early Dart SLFs disposed of by larger operators, however from experience it seems that many of these were only disposed of due to being completely sh*gged out, so in some cases they're not much of an improvement despite being low floor. Still, they do help operators move towards being DDA compliant if nothing else, until they completely fall to bits at least.
For a nice fleet of step entrance motors, all in decent condition, try Wilts & Dorset. With Go Ahead having taken over Southern Vectis/Solent Blue Line, much of their once large fleet of Olympians has been cascaded over to W&D, particularly the New Forest area opetations. Last time I was at Lymington depot there was a nice collection of all-Leyland Oly's parked up, along with a few of the Volvo variety with NC Palatine bodies, with some also to be found around the Poole area in the Damory fleet. The main W&D fleet also still includes some step entrance Daf DB250s with the impressive Optare Spectra bodies, as well as plenty of later low floor examples. Abus in Bristol have begun purchasing these upon disposal, plus at least one low floor example ex-Reading too.