My friend has very kindly given me her old iPhone 10 as she has a new iPhone with 02 under contract. I am also with 02 under payg and have an iPhone 6 which I have now backed up. I believe the SIM card fits the iPhone 10 so is it simply the case of taking it out and putting it into the new phone. Thanks.
Should be. You only have to do a 'sim swap' if you are actually changing cards. The same card from one device to the other should be no problem. I checked on line and they both use nano sim cards (apparently!), so should fit.