If you're looking for a decent price I'd recommend AutoNational Rescue:
http://www.autonational.co.uk/
However you need to be aware that only the AA and RAC have their own breakdown teams. All other services (including both Green Flag and Autonational Rescue) use local contractors. My experience of using the AA and RAC is that they'll always try to carry out a roadside repair before considering towing your vehicle to your home (or to a garage). Some of the contractors used by the other organisations simply arrive with a breakdown truck (rather than with the vast range of tools and parts carried by the AA and RAC), so that there's not much chance of a roadside repair.
Chris
PS: I'm currently with the RAC because I got free cover with my insurance policy. I had good service a couple of weeks ago when my car's head gasket failed. I used to be a 'trade plater', delivering cars across the country, with AA cover provided through the company I worked for. I frequently had to call them out and always received good service. My own cover was, for many years, with Autonational Rescue. Their contractors always arrived quickly (sometimes in vehicles displaying the 'Green Flag' logo, so they obviously use many of the same contractors) but often with little apparent interest in attempting a roadside repair.
Chris