Like was said, "officially" you should contact the DVLA,VOSA, get an SVA etc etc etc, whilst simultaniously opening up a large can of worms and also your wallet.
"unofficially" (as i was told by the SVA inspector who came out to look at my 90 twice whilst i was building it) they say "Hmmm.... you won't tell us next time, will you [-X ;) ;) "
Meaning, if you're just swapping the chassis, as you are, and your "new" one has no numbers in it (even better for you really), then just swap it, stamp them in and keep schtum... job done; don't advertise the fact you've done it to the masses, just in case there is some doo-gooder out there. But any officials would have a hard time proving when and where the chassis came from, so i can't see there would be any issue.
It's not like you're trying to rebuild a 1950 S1 onto a galvanised 2.4puma chassis (with puma bodywork) and claim it as a tax exempt :^o :^o... you're still willing to pay the road fund (arn't you? [-o< ) so the "crown" will be no worse off.