There are some kits that are manufactured to fit already registered chassied vehicles and others are creating hybrids with the (almost) complete mechanicals from one vehicle with a different body fitted.
As the basic component identity of the vehicle does not change, per DVLA rules , they would not lose their ‘right to registration’ and therefore would not require SVA /BIVA.
The ability to source vehicles via the Internet means that many are choosing to import ready-built modified vehicles at realistic prices.
Many may be spurred on by the statement that BIVA (Basic Individual Vehicle Assessment)only applies to post 1998 vehicles and that an older vehicle won’t be a problem.
If anything that appears to be too good to be true ever true?