Hi folks thought I'd drop in here with a few of my observations from the past couple months working on the other side.
As @Old Red Celica pointed out, the market right now is really tough, was on a webinar yesterday that was highlighting how motor insurance is effectively run at a loss - you can blame the roads being infested with hyper-complicated and technologically packed electric SUVs for that. The frequency of accidents has reduced, but the severity in terms of claims cost has gone up. As such, this is reflected in general prices, but you can still get a decent deal depending on the circumstances - many things are taken into account for the risk loading, it would be impossible to list them all.
The main sticking point with a classic policy over a standard policy will be the following trade-off: Do you want the insurance company to treat your car as an enthusiast, in return for less miles driven, or rather have more miles but have to potentially fight harder if the worst happens to get them to understand your point of view. I've also never heard of agreed value being offered on standard Private Car policies, but I am still learning this game.
For example, when my Volvo 480 was hit by the bin lorry a couple years back - had I not been in bed at the time and the claim was instead made against me, my insurance company would have likely taken the vehicle in return for my payout as it was insured on a standard Private Motor policy and that was what they included in their terms. As it happened, I got a payout from the refuse company's corporate insurance, and they let me rebuild the car, but it was a lucky escape for the wedge.
Generally, a 1999 car might be a tough sell on a classic car policy for an underwriter, but with proven club membership this should create a positive effect. If you find yourself looking for insurance drop me a line and I will have a look and see if I can chat to some people who will hear my arguments in favour of consideration.
At the end of the day, just make sure you carefully read the policy wording of any agreement and make a note of what you're covered for - comprehensive is a misleading term that gives the impression everything is covered, but there will be caveats such as one I see classic owners getting understandably anxious about, which is no cover on active airfields (think Bicester Heritage or Thruxton)
Hope this helps out, will be happy to answer questions
~Ben