It depends how big you are. If you turn over more than £150k you should deal with a commercial broker. They shop around for you. You don’t go to more than one broker, you’re appointing a professional to work for you, just like appointing an accountant.
There are only a few companies that offer business insurance plus a few schemes that are offered through professional associations. Once you reach a certain size the professional association schemes aren’t flexible enough and you need a direct channel to the insurers. That’s what a commercial insurance broker does.
it’s pretty difficult to buy commercial insurance yourself, there are quite a few pitfalls with exclusions and small print so if you aren’t big enough for a broker, go to a professional body, not your car insurer!