Suggest you contact Bishop Skinner, GJW and Haven Knox Johnston for a start. Plenty of others advertise in the mags. You will find that cover varies so you have to get quotes and compare.
If you want anything out of the ordinary, like 12months on a mooring, speak to a broker, such as FAstnet in Southampton.
check the cover is what you need.
Consider a larger excess, it may help.
Might be worth asking the owners club, saves repeating legwork, some classes have good relations with particular brokers.