In reply to Tobes:
I always used to slighly overestimate the mileage, but not by that much - but then I used to do my insurance through my Uncle (MD of a broker), so easy to discuss with him. He's since retired.
As already mentioned, it's worth playing around with the details on comp sites - I worked out that the sweet spot for my car insurance seemed to be around 4,000 miles a year. Any less than that and the algorithms think you're a very occasional driver and therefore higher risk as you don't get the practice - despite noting that I'm a named driver on my wife's policy. Any more than that and they figure that you're on the road more and therefore higher risk of being involved in an incident. Talk about fuzzy logic!
I'm not aware of any insurance company that's ever checked what mileage you're actually doing.
The other one to watch is not to over-estimate the value of the vehicle. Doesn't matter if you paid, say, £10k for your car 3 years ago, if it's only worth £6k today, insure it for that, as that's what they'll pay out. On the other hand, definitely don't under-estimate it, because they'll pay that, instead.
A long time ago, I spent a few years writing insurance broking software.