I'd only really use AF & Greenlight on a modded car, been with both and Greenlight for the past 2 years. prices were always pretty much the same, as was terms of cover.
Greenlight's £25 a pop for changes is a p***er, but track days at £50 a go is what makes it worthwhile for me, otherwise I'd be with AF as they do changes for free 99% of the time.
£470 all declared, full no claims and 32 years young.