Greenlight, Adrian Flux are hit and miss in my experience, sometimes they are great, but long term i found renewals usually poor - but this is probably fair to say for many insurers. (even if one year i was told by one of them "we could get it £30 cheaper with a different provider but its not worth the paperwork" - so i simply went elsewhere. madness.
Unfortunately there are so many variables that you just have to keep looking around, for one person a broker could be great and for others not so. they are have there own little areas and approaches that mean today they might encourage a certain cover type, of which there is car type, age, area, use as well as loads of other variables. sometimes quotes are silly purely due them not really wanting to push that area, its a polite way of saying we don't want your business, however if you are happy to pay silly money...
for me currently hastings are great. however i am older than most on here and i do not have a modified car. for those modifying / younger you will find different companies are better.
In theory a broker will be able to cut out the variances and give you a good price, but they are usually tied to certain providers.
Meerkats etc are fine, but they do cut things out of the policy to get higher up the results. often going direct can provide better quality cover. they will also tend to be best for standard type cars any variable and it might go mad.