In reply to droites: You can certainly get free PAYG SIM cards from Vodafone:
https://freesim.vodafone.co.uk/freesim There's no requirement to pay a certain amount over any given period, but if you don't make a chargeable call or send a chargeable text in a 90 day period then the account is put on hold, and after a further 90 days it's closed altogether (and you lose any credit you had). You can get an on-hold account reactivated by calling Vodafone's customer service number.
I keep a SIM like this in the non-smart phone I keep in the car in case of emergencies. I just have to remember to send a 12p text every so often (that's ~50p a year to keep the account active) and to keep the phone charged (mine retains its charge for months if I keep it switched off).
I'm sure other mobile service providers offer similar arrangements.