GiffGaff does not like you tethering, in fact they can get very upset about it. I think it might be available as an option now but it was the driving reason for me no longer being a customer 
Giffgaff says, near the bottom of https://www.giffgaff.com/sim-only-plans#accordion-8:
Can I tether with my plan?
Absolutely. You can tether on all our plans now.
Maybe the PR department updated the web page before the IT department updated the protocols, but tethering should not be the issue. And, as I said, the tethering worked for Safari and Google Maps.
Sorry, I donāt have any ideas here. Iāve only used tethering a couple of tunes for short periods and it seemed to work, but I didnāt extensively test. Does loading iCloud.com in Safari on the iPad work?
Just got back from a two-month trip to the UK and France. We switched to T-mobile because of the advantage over AT&T. Also used the T-mobile deal to get two new phones with a $700 credit (they credit you over two years).
Worked great. A bit slow, but GPS worked.
A warning for the lucky few who can do this: we got a warning at about week 7 that we were going to be cut off by T-Mobile for using data too long in Europe. We missed this in the fine print and it makes sense. The cut off was going to be after we returned. Apparently two or three months max.
ApplePay worked great on London Undergound. Both with pre-authorization or after touching. But pre-authorization worked betterāin other words bring up ApplePay and authorized (FaceID in our case) and then touch the contact area on the turnstile. Itās better because sometimes youād have trouble getting your face in the right place after starting the payment and post-authorization was always slower at the turnstile.
Havenāt opened our mail yet to see if any billing. We did make some calls.
I have Voipo for my home phone. They give me an hour of free calls each month to almost all calls in Europe and many others around the world. Some counties have their own carrier and charge like 50Ā¢ per minute, but most countries seem to be able to use the Voipo hour and after the hour, I usually pay a couple of cents per minute.
The bonus is that Voipo has an iPhone app, and I can use it to get those low international rates I get from my landline. Plus, when Iām traveling, I can answer my home phone.
The big issue with T-Mobileās plan is that I can text for free my Israeli relatives and get the 20Ā¢ per minute rate when Iām in Israel, but when they called or texted me ā even when I was just down the street, they paid their standard international rates. Next time I visit, Iām going to buy a second Israeli number from Voipo and use that. They can call that for their standard local rate.