Link says to keep original reg, must be original chassis. However, KW's Westfield is already Q plated I believe, so the rules look to change. Clear as mud though.
.. but you can have a "new and unmodified" chassis / bodyshell. In any case, that's for RAVs which would be things like hearse conversions.
With a kit you can keep the old registration if you have 8 or more points from the original vehicle (i.e. most of it bolted to a kit car chassis), you can have a new registration if the major components are new or "refurbished to an as-new standard" or you get a Q plate by default or if the parts are of indeterminate age. I was offered a new plate by the DVLA bloke because he reckoned I'd refurbed everything to an as-new standard but I declined because I'd have to suffer tedious things like emissions tests then.
But, in all of the above cases you'd need an IVA to re-register it. Replacing the chassis with new and NOT changing the VIN means it "never happened", so you don't.