Any modification to the car should be disclosed to the insurer. If the alloys were the same size as original but cost say an extra £600 it would make the car more attractive to thieves so insurers can argue that they need to be told and would refuse to pay out on a claim if it was not disclosed.
So everything should be disclosed to insurers the question is what will they look favourably on? Generally speaking if the rotational circumference is the same most insurers will not load the premium especially if you accept that if they are stolen they will be replaced with standard wheels. Some even allow wider wheels and larger dimensions but don't be surprised if they ask for an engineers report to confirm that the suspension etc is still perfectly aligned and setup!
