I have a SIM with Virgin and am currently halfway through a two year deal to buy my phone. Next year when I have finished paying for the phone my monthly bill (which is currently two payments to Virgin), will be just for the SIM which I have on a monthly contract and am able to switch between tariffs.
I have unlimited calls and texts plus 15GB of data (plus some other bits & bats from EE) - as I’m in contract and paying for my fancy iPhone handset it costs about £50 per month.
I’m out of contract in a few weeks time and my research says that I’ll then need to pay around £20 for the same package
You can buy SIM only contracts for as little as £10 per month depending on how many minutes you need. My last contract was sim only with Vodafone cost £9.50 per month with 250 minutes talk time unlimited texts and 2GB data. I've now got an iPhone with double that usage for £18 per month .
I had a two year contract with O2 which was for a new phone and when it finished they said I had to continue to pay the same amount a month (£21.50) or thereabouts. I went to Tesco and got a new contract for £7.50. Shop around.