Pay as you talk old style phone for an elderly person ?

[bobh]

Can I pop in a tesco .....o2 network ......sim card in that ?

Should be able to buy a P&G sim anywhere.
Tesco will be Tesco Mobile, I'm on that and it seems fine.
You can get a top-up card (top-up in Tesco only) or pay through the phone.
If either method is convenient, I'd go for it.

Just re-read your question.
The answer is yes.