The cheapest tariffs are from Lyca Mobile, Vectone, Lebara Mobile and Asda but these require the phone to be used more than the main networks (e.g. at least once every 80 days for Lyca). The main networks may allow a longer period of inactivity. However, I was surprised to be able to use a Lebara sim card that had been in a cupboard for a couple of years so the timing only seems to start with activation for those (while O2 sim cards get cancelled after about 6 months).
I have three phones and I've got into a routine now that I will make sure that I use each one at least once every month. For example, I make sure that one goes with me on nights out to book my lift home - obviously I would trust myself with an expensive phone on such an occasion!