My wife and I are both on GiffGaff and find it very good. Especially the fact you can call and text other 'GiffGaffers' for free - as long as you put a minimum of £10 credit on your phone every 3 months. Their £5 per month 'HokeyCokey' tariff looks very attractive and I may go for that soon. The only disadvantage is that if you have a problem, you have to go via their email messaging help system which can take some time. There is no help via the phone. However with this caveat, I would still recommend it.
What is Amazon Go and will it come to the UK? The store without checkouts or queues