You will not be able to drive without tax registration and insurance after an IVA pass so that would not be a goer. Insurance companies will cover you on a chassis number but only to and from IVA as well.
My car took 11 days from IVA to being registered and would have been quicker if I had had everything ready to go as soon as the pass certificate was in my hand. It was on the 11th day that I rang DVLA and spoke to Danielle Dymond who said, 'No problem , I'll find your file out and sort it out myself'. She did and emailed me the form at 4pm which allowed me to get number plates made up and on the road that day. Unlike some others I have always found DVLA very helpful if they can be.
Good luck with yours