I havent been able to find when the change to symetrical wheel plan came in but, it didnt used to be the case, you could register a 3 wheeler with a non symetrical wheel plan,
But I still dont think it would ever apply to a vehicle like this as the sidecar is clearly detachable.
I think if I was buying I would want a bit of information first, and do some checking.
On the whole I would tend to agree with Tarka though, as far as I can remember, the non symetrical wheel plan thing came in specificaly to prevent people registering outfits like this and get around the motorcycle and sidecar legislation.
The fact that its been registered, and is currently on the road does not mean that it was done corrrectly and that it couldnt be undone by sombody jobsworth enough.
Construction and use regulations 1986.
Three Wheeled Motorcycle - a motorcycle having three wheels, not including a motorcycle and sidecar.
And this is certainly not pre 1986
SVA Reg
Motor tricycle is a vehicle with 3 symmetrically arranged wheels fitted with an engine having a cylinder capacity of more than 50 cm3 if of the internal combustion type and/or a maximum design speed of more than 45 km/h.