With reference to the 10 year old term, it initially got me all excited, thinking my altered TR7 wouldn't need to go through IVA afterall, but the following extract from the linked document is key I think;
"In all cases, IVA (whether Basic or Normal) is only required for passenger cars and light goods vehicles
less than 10 years old,
which require first licensing and registration in the United Kingdom. "
So from the above you could assume you could import your 10+ year old Z3 from abroad, do your modifications and then register it without the need for IVA but if it is already a UK registered vehicle then you're stuffed!
This is just my interpretation of the above extract, it seems too big a loophole to be real
