I've been through that document sent by my insurance: https://assets.publishing.service.g...ical-interest-substantial-change-guidance.pdf and to me it is what is a substantially modified vehicle stills seems subject to personal interpretation. Especially that bit: " The following are considered acceptable (not substantial) changes if they fall into these specific categories: ... -in respect of axles and running gear changes made to improve efficiency, safety or environmental performance;" Reading this, one can argue that a Subaru engine gets better mpg and smoke test results. So it's an improvement on environmental performance and is thus acceptable. Same goes for Subarugears (better mpg). And red9 is a safety improvements. But doesn't really matter since I'm quite happy to have my bus tested.
Only if you are incapable of accepting that the guidance is open to interpretation in good faith and with sound science by the people issuing it. Whilst we are here...are you running original tyre specs and tread patterns? Why not? Because newer technology is widely available and performs better? Because the old ones were superceeded and unavailable? Weird that eh. Any thoughts on fuel hose suitability, or you enjoying modern petrol eating it's way through old spec hose? Maybe we should bring back asbestos linings on brakes? Didn't kill enough people first time round. That made me laugh on the day the site went down, and in each and every day since. For I am veritably the pied piper of the VW community. I declare all my changes to the insurance company and put it through its MOT each year, same as always. Not my issue if you have a chubb on wanting my bus and others to get Q plates so you can get some weird superiority thing going on.
I made the schoolboy error of trying to make a similar nonsensical arguement to yours in the hope of helping you comprehend my view point. I failed. As the leader of the rebel forces, do you want my reg number so you can report me to the man? Whilst we're at it...do you run a time warp van? Why do you care what I do with mine?