I don't know where you got that information from. What's the point of having an IVA process in effect in the UK if a vehicle doesn't have to be IVA'd after point of sale? As soon as a vehicle has been technically modified its type approval becomes null and void.
Here's an excerpt out of the guide to The Individual Vehicle Approval Scheme issued by the DVSA:-
It is an offence to use a vehicle that requires type approval without an Individual Approval Certificate(IAC) confirming compliance with the type approval requirements, except for the following purposes:
1. Submitting it, by previous arrangement, for an approval examination.
2. Bringing it away from such an examination.
3. Delivering it, by previous arrangement, to a place where relevant work is to be done on it following receipt of a refusal to issue an Individual Approval Certificate.
4. Bringing it away from a place where relevant work has been done on it.