I'm with Autoaid at the moment, but havn't had to call them out yet so can't comment on their service.
I think that the big boys in the breakdown world trade on and charge for their brand, rather than the service they provide.
Unlike the smaller ones who, having no brand can't charge much, and actually provide no service when you actually need it.
I've been with 3 breakdown companies.
1) RAC - 2 callouts. 1st left me for 3 hours in the dark on a twisty A road between Henley and Reading, despite me telling them that I was stranded in a dangerous spot.
2nd broke down in Bristol and they told me that they couldn't get anyone to me for at least 5 hours!
I told them not to bother and walked home!
2) Rescuemycar.com - 5 callouts over 9 years membership. Each time someone was with me within an hour. Was recovered home 3 times without any quibbles or aggro.
I paid about £19 a year premium plus 5 x £40 callout excess, which works out about £41 a year. A massive saving on the Big Boys of the breakdown industry!
Unfortunately they won't accept cars over 16 years old which rules out my BMW.
3) Autoaid - Been with them for the last couple of years, but havn't had to call them out. I think the premium is about £40-50 for any car cover and there are no age restrictions.