Also you can't have someone turn up in a Foden steam wagon for an MOT
Actually.. I know of at least one Sentinel where a curious owner persuaded an MOT garage to attempt to stick an exhaust gas tester in the funnel. It melted. I'm pretty sure that's not just one of those stories, although I can't remember who it was that did it.
The DG6 has long been exempt. There are oddities with exemptions.
The ban on solid tyres exceeding 12mph was, for years, in name only. It's written on the cab sides 'Max speed 12' - it's more like 30 (18-20 'cruising speed' according to the Garmin). I think that is now lifted, actually. (Side note - there are 'shower heads' over all 10 tyres as solids can get hot and turn to gel inside).
It's hefty but can be driven on a car licence. (Unlike a roller which requires a road roller licence). I have never known that to be an issue, not many Sentinel/Atkinson/Foden drivers are too keen to damage them for fairly obvious reasons. It would be prohibitive to some owners/drivers if that changed and their insurance (road) is cheap presumably for a reason. As a driver of vintage commercials it is not always in our own interests to rock the boat and remind the powers that be that we exist. The only Sentinel fatality I know of involved the vehicle that is sitting 100 yards from me right now, and was the fault of a car driver attempting to fit between it and the HGV/LGV towing it - years ago now, and under previous ownership, but neither caused by or due to the nature of the Waggon itself nor its age. Same result would likely have happened had it been anything on-tow. By the way they do have particularly good handbrakes.
On the MOT/Tax topic, I have amongst other things an older (than me by 2 years) Citroën which became MOT exempt in 2017. It is in need of some welding, and the brakes need work. Even if it is likely a 'minor' point, I would still like to be able to book it in and have it given the equivalent of an (age appropriate) MOT.
There are lots and lots of debates in vintage/steam/classic/you name it circles -
I have heard the point that not many garages would like to put their name to that, without actually MOT testing it, but that makes little sense - if I took the P38 for an MOT and then cut the brake lines round the corner, that is my problem and not the MOT centre's.
There is also an argument that tax exemption could lead to a future 'well you don't even pay road tax [vehicle excise duty] anyway' type argument if the topic of banning older/petrol/diesel etc ever came up. In truth, if they were to actually do that then they would do it anyway. Many classes of vehicle are tax exempt, not just classics and vintage.
EDIT to add: Re. the Bentley example, I actually got handed a Boat-Tail Auburn to work on back in the late 90's when I was a teenager in a rural garage. 2 speed rear transmission was jammed. Can't imagine that happening today. We had some fun cars come through there from time to time.