Look carefully at the terms and conditions.
The major brands are dearer, but usually for a reason.
Take uninsured driver cover. The brokers don't cover this. If you get hit and the other driver is uninsured, you pay the excess, you have a fault claim against you that stays for 5 years.
Also, accident service. Some smaller companies only operate claim lines in office hours. If you have an accident in the night, you are stuck till morning. Same on recovery, check that recovery in the event of a fault accident is covered. A policy I had done years ago had a fee structure to pay in event of recovery being required after an accident.
Final point, the smaller firms don't have the speed of the big boys.
About 2006 I had a policy with Swiftcover when it was a new company. I was hit by another car and the other driver admitted responsibility. The other insurer delayed the pay out for three months. Several calls to Swiftcover revealed this was common. Other companies withheld payments to companies they don't have a trading agreement with.
Just saying. Check the paper work, and check the basics against the mainstream competition. On some policies, the basics that you take for granted aren't covered unless an additional fee is paid. This gets the quote to the top of comparisons, but yields a very basic policy.