Try wording it differently to your insurance.... it may sound weird but it often works. Dont phone up with "I want to put some new, non standard alloy wheels on my car...."
Try something more like this....
"Hi, I managed to hit a pot hole last night and burst my tyre and destroy my alloy, the price of a new renault set of alloys is over 500£'s but i have found a set which are the same size going on the internet for just over 300, i was wondering if there would be any difference to my premium".....
I know it sounds weird, but it worked a treat for me, and that was to go from 15inch renault alloys to 17's!!! Just claimed i could get the 17s REALLY cheap... Only added 25 quid for the 6 months left on the policy, (so woulda been 50)