Excuse a stupid Swede for asking, but what is a BIVA test?
Our wonderful Governments latest wheeze to create as much grief for law-abiding citizens as possible........
Basically, if you modify your car to a large extent (and chopping the roof on a monocoque car is considered a large extent) you need to go in for an inspection which costs a significant amount of money and requires you to abide by many rules in order to pass. BIVA isn't something I'm against (we have it better than a lot of countries - try building and registering half of the cars on RR in Germany, for example!) but it is a costly and bureaucratic thing.
I can't remember the last time I saw an Ital. Personally I don't think roofchops work on square-ish cars like the Ital, but that's just my taste! Good luck with it whatever you decide to do.