I declared more than 5 mods to my car the other day, including alloys and cobra classic seats, didn't even have to pay an admin fee! They did say when I first took out the policy that if I put alloy wheels on the excess would increase by about £100, which I thought was fair and I was expecting them to say that when I gave them a call. Couldn't believe it when she said that's all fine
As for the stripes - I'd tell them about it although if you're worried about an admin fee again, wait until you have something bigger to declare. It's hardly a modification, but I suppose it could make it more of a target because they make the car look that little bit better. Plus we all know stripes make you go faster
"when you ring just say you got a set of alloys and you want to get a quote to see how much extra your insurance will be, that way if the price is affected loads you don't have to go along with it, you can just make an excuse so they don't make you pay anything." ----- Is that really a good idea? I'd rather pay the extra because you can guarantee if you have an accident, the insurance will say you're not covered because of the wheels.