My classic (?) Spamcan turned 50 this year, and much of the UK fleet probably isn't far behind! Unaware of any specialist insurers, but it's worth trying several for quotes. I won't promote the people I'm with as it probably isn't allowed, but they were very helpful in matching the best price I could obtain from a relatively recent newcomer (to UK) that does everything online and is very competitive. As far as hours are concerned, you'll be asked how many hours you expect to fly (and how many last year, and experience on type and overall, hull value etc) and that decides premium. Because you are subject to a maintenance regime, there should be no restrictions on hours, just how much risk you represent.
Do try a few as quotes can vary dramatically! If it isn't obvious who the online guys are, feel free to PM me........