I swapped the 19s for the 18s with winter tyres today. I phoned the insurers, Admiral, to let them know I'd swapped them over and guess what. They wanted to charge me a £19.50 charge for an alteration to the policy.
I tried to explain that this change was making the car safer during winter. Computer said no.
Incredible.
Most insurance companies these days don't require you to tell them if you're on winter tyres as long as you don't run non-OEM wheels in the process, so i've stopped bothering.
If you are going to tell them though, it's better to tell them at the start of the policy that you'll be running winter tyres in the winter, otherwise you may be giving them an excuse to charge another £20 when you tell them you've changed your wheels back again.