You may be able to get away with just registering it, this page explains what is required:
http://www.direct.gov.uk/en/Motoring...le/DG_10014199
However it depends on being able to prove the components came from one vehicle.
Failing that you may have to register it as a "kit conversion" in DLVA speak. In which case you must go through IVA (the sucessor to SVA). The reg you will get depends on how many components came from one donor car.
IVA is more stringent than SVA and I would suggest you contact Blitz and ask if any vehicle has passed IVA, and if so what modifications were needed.