Semester abroad son in London needs to find the best in value and the best in service pay as you go SIM card for his cell phone. His USA phone will work in Europe but looking for a cell phone provider for monthly top up pay as you go plan. He will need some international service but will have his home phone SIM card for that so really looking for an unlimited UK text, at least 2 Gig data plan and minimal UK call package. Would appreciate suggestions for a reliable provider with an economical package for Pay as you Go SIM plan? Any thoughts on GiffGaff provider? Also looking at Vodaphone and O2 -- Any others?

My student daughter had a sim from Three. I use Three when I'm in the UK. Good value, easy to use.

For international calls we use Viber or Skype - since he'll have internet on his UK sim, no need to make expensive international calls. And use Whatsapp for texting.
The kids don't use text messages at all, just Whatsapp.

Just checked, we use the 15 pounds/month unlimited data package, but you can go cheaper, from 7 pounds/month.
Other providers offer similar deals, but frankly it's such a minor expense compared to living in London, that I wouldn't spend too much time on it. Just walk into any Three, Vodafone, O2 shop and get the right SIM for the phone you have.

Look at https://giffgaff.com/
Very popular among youth, as many of their friends are on giffgaff and it's free to call.

Check what Tescos has at the moment, normally a good deal