Aim Far
You've got to be EXTREMELY CAREFUL when getting aircraft insurance quotes! (Let me just say I learnt this the hard way some time ago)
In the UK, all GA insurance is done by ONLY approximately four Lloyds underwriters, and one insurance company which I believe is currently called BAIG or AIG.
But there are maybe fifty brokers. These are easily found via the internet and some advertise in the UK flying mags. A lot of them are rather "part time" operations.
If you want to make yourself extremely unpopular (uninsurable), write to them all for a quote
This isn't a competitive business like e.g. car or domestic insurance. This industry seriously dislikes people who shop around.
You can't go to an underwriter directly. I am PM'ing you with some details.
Unless you are doing something unusual, there won't be a lot of price variation. And if you get a low quote from some underwriter and later the same day a request from another broker (for the same plane) lands on his desk, he might just up the price. I saw a price go up 3x on one occassion. So you've got to play this carefully.
If you really want to just get lots of quotes, use a fictitious aircraft registration (check on G-INFO it doesn't exist first) but to be honest that isn't a good idea because you could make somebody else's future plane uninsurable instead. And some numbers are "taken" but don't appear on G-INFO.
Discuss carefully with your broker what your options are. For example, if you will have other pilots, "named pilots" is a lot cheaper than "club use" but is practically the same thing if you follow certain rules.