I have a Volvo estate. It was supplied new with 17" alloy wheels which take low profile tyres. Volvo always offered the options of 16" and 15" steel wheels, to take tyres with wider walls. I seriously considered swapping over to one of these options because they are more robust against the state of the roads, due to the number of potholes. A single new wheel for mine would be over £300.
I spoke to my insurer about doing this and was told it would be classed as a modification and would add £30 a year to my premium. I pointed out that since these were an original OEM option there was no way they could tell which wheels were originally fitted, and that I was considering doing it to reduce the likelihood of a claim through damage. I had a look at the questions around this 'modification'. They were questions like 'have you modified the width of the wheel arches?', 'have you increased the turning circle?' so clearly not aimed at what I was talking about, but in the world of computer-generated premiums it was a modification and that increased my risk and hence my premium. In the end i picked up an absolute bargain set of spare rims on Ebay for £100 and 7yrs on I have used all of them.
My neighbour opposite used to run a transport business as owner/driver. One of his vehicles was a small flatbed hi-ab and one of the jobs he regularly got was removing irreparably damaged cars from the M11. One time i went over to see him there was a smashed up Golf on his drive. This was unusual as they normally ended up at 'repair' place in Haverhill, before going to the scrapyard. This one had come to his place as the owner wanted it back but the insurance assessor was declaring his insurance invalid - he had fitted standard size but non-OEM alloys (to replace the original steel wheels) and a wider bore stainless steel tailpipe on the exhaust, and not declared them as 'modifications' which the insurance assessor regarded as invalidating his insurance. I never did find out who won that argument.
Neither of the above relates to a classic insurance policy but both do indicate how picky some parts of the insurance system may choose to be.
Alec