My understanding is as follows :
There are limits as to how far you can change vehicles from their original design before you must change the registration and put it through a single vehicle approval (SVA) test. (You would get a 'Q' plate)
There are no limits as to what you can do, other than the construction and use regulations which govern the design of all vehicles on the road in the UK.
Technically speaking, many of the vehicles driving around our roads are not legal, as they should have undergone SVA. The Police / MOT don't seem to mind too much, provided you are not claiming Tax Exemption ilegally and the vehicle is fundamentally sound.
Details of the vehicle modifcation 'points' system which governs SVA requirement can be found on the DVLA website.
