Just make sure you know what they cover in the small print.
I paid loads every month then worked out they changed timers pumps valves thats all that went wrong on mine.
I've cancelled it three years ago and did the work myself which came to less than three years of repairs!
But I know my boiler well and its good even service it myself usually means not touching anything gas related, but have done, fan, thermostats, valve, pumps, immersion heaters, PCB, Pilots and pipe work Still ended up cheaper than what I've paid for cover!
I'm getting the loft insulated on monday and went out bought some insulation for the cold water pipes and tank. It came to about £40 £20 of that was the sh!tty plastic lid, but you are right preparing this winter to come is not a bad thing considering getting a pumber @ Christmas is a bloody nightmare!