ANE went fully low floor before both GNE and SNE iirc (yet another thing that is rarely brought up because it doesn't suit agendas). As noted above the only vehicle it's likely to have been is 4015 (K415BHN), but that's not used in public service. Anything else K reg would have been long gone - M reg Olympians went in 2010 for context.

I would disagree with this.  Stagecoach Hartlepool was first depot to be fully low Floor after the Spring 2010 service changes when the 2 remaining Volvo B10 where withdrawn from service with only enviro 200 and Darts been used.   I know Arriva and Go North East still used step entrance buses for a good few after that.

As for the pros and cons of each of the big 3.   

Well Go North East have the nicest buses but where I am they kept changing the service number, adding services and withdrawn it after a few years and now they don't have a service here at all.    

Arriva buses are mostly ok. They seem to have a stable network at least and rarely about with services.    

Stage coach. Main operator.  Buses are rubbish and frequently breakdown.   Service changes have made services worse in some areas and no investment in the fleet.    So I would say none of them stand out they all need to improve.