Hmmmm....Asking the cops for clarification isn't always going to produce accurate info!
For instance they talk of a bike built 'from lots of different components from different bikes'. But this could mean a 'special' or a bike built from standard parts from the same model, which is basically the same as a resto.
Clearly the latter isn't going to be appropriate for an SVA test as this would require a lot of inappropriate parts; indicators, anti-theft devices, idiot lights etc.
In his 3 examples, the first with its '63 engine is going to need an MOT as they wouldn't register it as pre '60.
But the other 2 could be exempt. The DVLA will register his '55 basket case as a '55 if its year can be proved. Similarly the '57 RR.
To illustrate; I have a '59 Chevy pickup. Bought it in LA in '91, imported it, restored it and registered it in '05. Its on an age-related registration and is MOT exempt.
Bottom line is, if its registered as a pre '60 bike, its tax exempt.