There is a strong possibility that the ITV examiner will require you have you rewire the rear lights so that they operate properly, and that may require you fitting replacement light clusters and extending the respective live wires from the fog and reversing lights over.
The ITV staff often take a 'it's more than my job's worth' attitude to these matters. They fear that if they approve something one year, that another examiner will make a formal complaint about that something the next year when it goes in for it's next test. I had a similar problem when I tried to get my car through to put it on Spanish plates, over the headlamps. The dealers had fitted new glass lenses which they claimed met the ITV requirements, but the ITV station refused to pass the car. The dealers' manager then personally took the car up to the ITV station and had a very animated discussion with the ITV manager who admitted that it was probably not a problem, but he was concerned that another examiner may take a different view and make a formal complaint to their bosses to the effect that the car should have never been given an ITV.
It cost a fortune to get the car through, but the only reason I did it was because it was a classic. I cannot see the point otherwise. In fact, there used to be a couple of guys that I knew who imported cars from, mainly, Belgium and Germany because they could pick them up for peanuts there, and nothing was needed, apart from the paperwork and a straightforward ITV, to put them on Spanish plates.