Go to this site:
http://www.rac.co.uk/web/breakdowncover/uk/
Half way down the page you will see a paragraph headed 'Broken Down?'
They give you a freephone number to ring, and a reference number to give, if you have broken down. You will get Roadside assistance sent to you, though obviously you will be 'invited' to join there and then.
You would need to check regulary that this promotion is still on offer, making sure the reference number to quote hasn't changed.
My breakdown cover ran out about 3 months ago, and currently I'm using this method to be 'covered'. So for this period it hasn't cost me anything.
In the 3 months I've been using it they have not changed either the telephone number, or the reference number.