IIRC you can replace like for like without incurring a penalty, but you CANNOT use a second hand chassis, it must be brand new.
I copied this from the DVLA website:
Your vehicle must have 8 or more points from the table below if you want to keep the original registration number. 5 of these points must come from having the original or new and unmodified chassis, monocoque bodyshell or frame.
Part Points
Chassis, monocoque bodyshell (body and chassis as one unit) or frame - original or new and unmodified (direct from manufacturer) 5
Suspension (front and back) - original 2
Axles (both) - original 2
Transmission - original 2
Steering assembly - original 2
Engine - original 1
Get a ‘Q’ registration number
You will not be able to keep your vehicle’s original registration number if one of the following applies:
- it has fewer than 8 points
- it has a second-hand or altered chassis, monocoque bodyshell or frame
- there’s evidence that 2 vehicles have been welded together to form one (ie ‘cut and shut’)
Your vehicle must pass the relevant
type approval test to get a ‘Q’ prefix registration number.
