I would wait til I get near to mileage on policy and then tell them just in case something changes - they may not bother too much as some insurers operate in bands - eg 10 to 12000 - they seem flexible below 15000. Sometimes you are asked to porvide details of mot mileages as said or service records in the event of a claim. Maybe ring up and just ask the question - anonymously if possible
A mate with a kit car did just that. A couple of weeks before the policy ended he needed to extend the mileage by a couple of hundred miles to do a trip away.
He was told "You'll have to upgrade the policy from 3,000 miles a year to 6,000, that'll be £<3 figure sum>".
"But....

the policy has only 2 weeks to run".
"Doesn't matter. Next step up from 3000 is 6000".
"OK. Sod it. I'd like to cancel the policy and start a new one for 6000 miles, then".
"Can't cancel it with less than a month to run".
"OK. I'll just take out a new one, then".
"Can't have two policies covering the same risk simultaneously. 'puter says no".
"O----K. So, what's stopping me from going to another company and starting a new policy?"
"Nothing, sir"
"Bye..."
