It's the rebuild costs for period properties, they've gone absolutely insane. In Spring 2023 I got quotes to insure a 4 bed stone cottage we were thinking of buying, it had an asking price of £465k, the insurance company were quoting a ONE POINT ONE MILLION POUND rebuild cost. The quote back then was 1.3k PA, but she also made it clear 'we're not done yet, we need a load more details off you and it's likely it will go up significantly because I only have the basics off you.'
(we didn't buy the house)
Because I was exceptionally pissed off, I started playing around with quotes, and got a quote for the rental we were living in (both buildings and contents) which was a 12 year old brick built house, with a value of around 600k imo (it was huge - 2500 sq ft), the insurance companies were quoting a rebuild cost of 386k for THAT house and I can't remember what their quote was, but it wasn't in the slightest bit disturbing.
Anything period is basically through the roof because the cost of materials: wood (and stone) has shot through the roof since covid/brexit. As have labourer costs.
Go to the Facebook group: Your old house UK - repair and conservation - where this is a frequent topic of discussion, and find an insurer through whomever has been recommended there. You'll probably find a fair few replies 'I'm a broker, message me' and you can try it that way too.
Good luck. It's very expensive to own and maintain a period property these days.
Edit to add: as we hadn't owned a home before we're likely to have to pay more, but last week I got a quote for a 415k grade ii listed 4 bed Victorian building and it came out at about 350 PA (at the cheaper end), I think the rebuild they were quoting for that was around 390k.
There can be 100 years between Georgian and Victorian properties though.