In my case, they took my word for it. That "uberimae fidei" (utmost good faith) thing cuts both ways. HOWEVER .... insurers do have loss adjusters, claims investigators, fraud teams, etc so in the event I stick in a big claim, they can investigate where I was and why, and if it eas a businrss trip, and if so, the extent of my business use.
Personally, for tax reasons, I keep a running tally of business usage, and each trip is diarised (the paper kind, not on a smartphone) so I can back up when I was and wasn't using business miles.
I suppose, in my case, it wasn't just simply a case of saying " I do x miles" but rather more a case of explaining semi-retirement, might not do any business miles at all but would like to retain the option to do a handful per year. On the basis of an average of 150 miles each way, and 3 or 4 times a year at most, it shouldn't exceed 1500 miles, but as I keep an eye on it, I aas told "if it does look like going over that, let them know, shouldn't be a problem.
In fact, it's rare enough now I could probably do without any business miles but as it's costing nothing, and keeps the option open .... why not?
It is, I'm told, all about perceived risk. Givren my circunstances (no claims in a couple of decades, no convictions, age, semi-retired, etc,) I guess they see no extra risk in the few business miles. If they did, they'd price it in.
Also, it's kinda hard to customise like this if you get electronic quotes via a comparison site. But use a good broker you can talk to, and they can negotiate a variation direct with the insurer.