Ok, I've done it with motorhomes in the past it's fairly easy but a little more expensive now than before.
You need to pay VAT on anything that comes in now, no difference if it was a gift, cast down from the heavens or you got it for free ...VAT and duty on everything.
You should pay VAT and maybe customs duty at the border - ......'Any motor vehicle over 30 years old is automatically eligible for import duty relief and a reduced rate of VAT. (You will usually be asked to provide documentary evidence, such as a copy of an insurance letter/certificate stating that it has or is an offer for 'classic car insurance')'
You then need to apply for a NOVA certificate....thats a five min process.
Then get a UK MOT on it, this will be based on VIN number - no need to change the speedo and on most bikes just mask the LHD headlight kick-up pattern.
Fill out the DVLA registration forms, pay the fee and send the NOVA details, and MOT details to them, then wait a few weeks and you will get a V5C.
No need for insurance or anything like that, but you can insure on the vin, supply that at the same time to the DVLA and they will tax it too at the same time.
I know people who bring bikes back in vans and do not declare them at the border ...but why take a chance ....border security and the customs and revenue are not people to mess about with, once you faff them about you will get stopped every time and HMRC have almost unlimited powers to investigate you.
https://www.gov.uk/i...ng-vat-and-duty