Klarky, if you don't have the certificate revised, obviously the ex-works kerbweight will continue to apply and all accessories and later fitments will come out of the payload. Usually it will not be worth the effort to have technical changes documented, but on the other hand, if you were to purchase a van and later convert it into a motor home, the difference will be substantial enough to warrant going through the process.
You are partially correct, Ray. Retrofitting of alloy wheels is a technical modification, but cannot be classed as a permanent one as it is very easy to convert back to the original wheels. Theoretically, one could have the kerbweight revised to reflect alloy wheels, but then there would have to be a note in the V5c that the vehicle must only be fitted with such wheels. I find it difficult to imagine that the powers-that-be would accept such an amendment. Therefore, changes to the vehicle weight in this case will normally come out of the payload, as mentioned above.