Of the top of my head I would say minibus limits.
If you want the regs they are
here
It appears the vehicle is classed as having been 'adapted for carrying more than 8 passengers'. (It also appears that the term 'adapted' includes 'constructed' as there is no such separate class)
The limits therefore appear to be the same as a minibus. That is 70 60 and 50.
Note that buses longer than 12 metres are restricted to 60 60 50.
I believe the 'adapted' term rather than 'constructed' allows you to remove seats, as mentioned above, to change the class of the vehicle.
PS Not sure how the speedlimiter regs affect such a vehicle but if required it should be restricted to 62mph
PPS shouldn't this thread be under 'Legal/Legislation'