Have you thought about a secondhand one Andy. I bought one from these people and have a GiffGaff sim card for £6 monthly. Unlimited texts and minutes and I'd have to look up the data, but it's sufficient for you I think. Oh and the phone was like new.
https://www.thebigphonestore.co.uk/refurbished-phones/iphones?route=common%2Fhome
If you want to stick with Tesco but need the cheapest current iPhone (which should easily meet your needs), rather than considering an older refurbished phone, Tesco Mobile has a deal where you pay nothing up front for a brand new iPhone SE 2020 but an extra £9.99 per month (for 36 months) on top of your normal phone tariff. (So the phone works out at £359.64, which is a good price for an iPhone anyway):
https://www.tescomobile.com/shop/apple/iphone-se-2020
If you’re on FB, look for your local phone repairer - they occasionally sell on phones that they’ve refurbished for a fraction of the the price of other refurbished ones (and they don’t want the grief of selling a duff phone and getting slated on FB).
I switched to Tesco this year and went with the iphone SE 2020 as suggested above, and am delighted with it. The transaction was smooth and efficient, the phone was straightforward to set up and the service is reliable.
It will cost £17.99 pm (£8 for data and £9.99 for the phone)
Any questions about it just ask.