If you're considering changing to basic PAYG then giffgaff rates are quite expensive.
I'm trialling 1p Mobile for an elderly relative. It operates on EE's network and minutes, texts and data are all 1p each.
It was easy to set up, great signal, itemised usage, works with older 2g phones, autotop up if you want it, can fund via PayPal, UK based customer support. You can roam in the EU at same 1p price, including Switzerland, Isle of Man and Jersey, (we'll have to wait and see if Brexit changes the price!). International calls start at 9p per minute which is not extortionate either. So far, so good!
Its not great for a backup phone or a very low user as you have to top up £10 every 120 days which works out at £30 per year/£2.50 per month.
The next cheapest are O2 Classic and Three PAYG who both charge 3p/minute, 2p text, 1p/mb.