Along those lines, on older Android o/s ( marshmallow)it may be useful to go into "settings" "general" "battery usage" and see what apps are on and using battery.

I noticed yesterday that Gas Buddy had used 20 percent of my battery, even though it wasn't open in my app window. From that screen I was able to shut it down. Presumably it is also using data while it is running in the background.

I also see that you can go into each app while under "mobile data" and restrict background data except under wifi. That's probably worth doing on data hungry apps.

Thank you Andrew. I appreciate your help with this.

I brought up Three because that was what a UK friend suggested when i asked her about PAYG SIM. That's the one she buys for her visiting relatives. But thanks for mentioning about coverage. I emailed our host in Dorset and he said coverage is iffy in his area and he uses O2 which is okay.

Having gone through all the answers I think in the end, I'll just take my chances and wait until I get to Heathrow and hopefully will find a store to sell me a SIM at a cheap price!