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The IVA area is contradictory in places and very definitely designed to minimize car modification with regular updates to keep pressing down on our hobby.
As Paul has said , Hot Rods for years have been built and registered incorrectly . IVA or a version of it has been in place since the 90's but they have tightened it up in more recent times.
If VOSA chose to inspect any of them , or they were to be flagged up at MOT , they could be called to be inspected ..its going to happen more and more as time goes on .
Fitting a major subframe to your chassis WILL be an IVA issue whether bolted on or not.
I used to make a very small bolt on adaptor to fit a Beetle chassis years ago to reposition the front axle. I was fully within the regulations at the time but somewhere along the line someone questioned it with DVLA . DVLA traditionally err on the side of caution if they don't know the answer or find their regs contradict themselves , so they immediately reversed their decision on it being satisfactory and outlawed it by changing the wording of their regulations.
I worked quite closely with ACE when creating the Sammio range and my first 2 versions were deemed unsuitable although no major chassis mods had taken place. My internal framework was welded in place and not bolted on . This was deemed a major chassis mod at the time although no chassis spec had been altered . We had to bolt it on and bond it to the body in the build process so the body can be removed , technically , with the frame attached to it ...this deems it to be body bracing and not chassis bracing ...its all in the wording !!
The second issue was the rear outriggers . They serve no function at all in regards to structure and if you read the rules , removal or minor body mount modification is acceptable ..DVLA decided that removing the outriggers at the rear was a step too far , they can however be trimmed down in size .

A lot of people bend the regs or interpret them differently , but its important to be aware they aren't stupid and can reverse your decisions or put prohibitions on the worst cases . I know 2 Hot Rods in the last 15 years that have had prohibition notices on them , so its rare but it happens .
There was a local ish vehicle inspector who had a passion for Classic VW's ... he hated modified VW's he used to attend several shows OFF DUTY but make notes of reg. no's of vehicles he deemed to be illegally modified , primarily Beach Buggy's built recently on SWB pans and owners would get letters sent out by DVLA as a result ...He also was instrumental in Chesil having issues with the way they registered their 356 Speedsters a few years back and having to make changes ...so don't under estimate 'The Man' ....