Do not worry.
Insurance companies can only decline a claim for undeclared modifications if the modifications directly affected the risk. In this circumstance,having alloy wheels made no difference to the claim event. If you had a performance enhancement added, e.g a turbocharger, then maybe the Insurers could make a case that the performance enhancement could have affected your ability to control the car.
Admiral will deal with the claim as normal and you may be asked to update the details to include the allow wheels as a mod. You will be asked for the approximate value of replacing with similar alloys. This is so that Admiral have an idea, should the car and/or wheels be subject to theft. When Admiral amend the policy, don't let them charge you an admin fee. They normally will charge a small fee if policy details are amended, but if you explain they should be willing to waive fee.