What you really need to know is which insurance groups the cars, which you might be considering, fall into. (Although insurers aren't obliged to follow the 'group system, they almost always do so. i.e. a car in a low insurance group will nearly always have a lower premium than a car in a higher group).
There's an excellent website where you can simply input the make and model of the car to find out which group it falls into. Even better, it also allows you to select a group and see all of the cars in that group:
http://www.parkers.co.uk/insurance/Groups/#
There are plenty of brokers who will claim that they search all available policies and can get you the cheapest deal. Don't believe them! Try phoning local brokers to see if they can beat the 'big boys'. For example, when I bought my first car, the AA told me that they'd checked lots of policies and could offer me the cheapest deal. I phoned a local firm and they offered a figure which was nearly 40% cheaper!
Chris