IMHO, agreed value is more important than much else in terms of insurance of older vehicles, given the well known proclivities of insurance companies..
We're with A-Plan - no affilation, and no claims either, which is when we'd find out how good they are.. Suffice to say we used to be with NFU, and IMO, they are utterly useless..
Classic can be better and cheaper, however the premiums tend to sky rocket if you do more than 2,000 miles. We just insured using a regional insurer rather than a 'classic' specialist, end result same premium except we are allowedf 7,500 per year. Although used mainly for driving in London, I thought what the hell, what if you wanted to pop over to France for a booze cruise? You could use up between a third and a half your annual allowance in one hit!