The iPhone has a reputation for being a well made, easy to use device. It's rarely known for being a cheap phone though.
However, iPhone SE pay monthly contracts now start from as little as £13 on the O2 network. For this you get 500 minutes, 500 texts and 1GB data per month. However, this offer does include £96 cashback.
The cheapest non-cashback deal we know about is on iD mobile with 300 minutes, 5000 texts and 500MB per month. It's £12.99 a month, but has an upfront phone cost of £60.