I think it depends on circumstances. If your boat is quite new, kept in a marina on the South Coast, they may be competitive. When I had a boat that was 20 years old, kept on a swinging mooring in North Wales they were horrendously expensive, even with RYA Boat Show discount.
Haven Knox Johnstone have been excellent for us. They were the cheapest quote we received, have never charged extra for cover outside the standard cruising ground, always respond immediately to questions and don't request unnecessary surveys.
Look in the back of PBO, Yachting Monthly or Sailing Today and you'll see all the major ones listed.
I'm with KC Powell & Partners - Tel No 01702 470035
Phone Phil who is the Commercial Manager at The Insurance Centre in Norwich tel 01603 403637. They run a number of yacht schemes and everyone is very friendly and helpful.
I have been very pleased with service over 10 years.
1) You will see that a number of insurance offers go back to the same backing companies (like Sun&Alliance) although marketed under the brand of the brokers.
2) It depends regularly on the detailed circumstances whether a company is cheap or expensive. Size matters a lot, but so do marina/mooring, cover ashore, included personal effects, sailing areas... and there is no easily predicted pattern: The same company maybe great for a new big boat in a marina, but worst for a liveaboard in the Med.
3) Attention points: Clubracing / club rallies, temporary mooring (for longer periods) in different circumstances, cover ashore -> because that's where a lot happens and some companies renounce cover (AFTERWARDS)
You probably have to go through the procedure and get offers, unless you find someone with a sisterboat at your place with your cover needs who has good experience...
More important than saving a few quid is a broker that is on your side!