When they originally started to go down this road there was an outcry from the classic car community.
They then had consultations and clarifications with the FBVHC.
They clarified that there is a points system to consider and assess what is Radically Altered! Your car must score a minimum of 8 points
How to register your vehicle and the cost - new registrations, kit cars, rebuilds, radically altered vehicles, old and classic vehicles, vehicle identification numbers, Q plates
www.gov.uk
Now then a further clarification came - for example an engine swap (say for a non-L series) does not lose you points providing it has the same configuration I.e. a straight six.
But let’s say you have leaf springs and change to coil overs then you lose points on suspension.
On that note, this is one of those things that I have to consider when writing dating letters. I have in the past refused some cars due to radical alterations - such as cutting away the spare wheel well area to make way for a racing fuel cell combined with a V8 and so on. But if you have the techno toys tuning suspension arms, you haven’t changed the configuration.
Once in a while we get stories of extreme situations because there is discretion on behalf of the inspector / assessor and there are often good reasons to their judgement - but 99% of the time our mods are within the rules to avoid a Q plate.
Edit: something to consider - if you add a roll cage and it goes through the firewall or requires cutting away of any of the monocoque then you’re loosing the chassis points!
Hope that helps.