Very easy. I have a UK sim with Giffgaff. I lost the original sim and ordered a new one (not linked to me, it was a freebie for distributing to get a referral).

Very easy. I have a UK sim with Giffgaff. I lost the original sim and ordered a new one (not linked to me, it was a freebie for distributing to get a referral). I could log into my account, activate the new sim and it was done in about ten minutes. That's really nice in some respects, but in theory if your account gets hacked, goodbye phone number. There may have been some additional work involved, eg confirm via email, but I believe other networks make it a lot more difficult. Eg you need to request it specifically through customer services and they need actual ID. My mum is with EE and she had to go to a physical store and prove she was the account holder. Three is similar, you have to request a new sim but in theory if your mobile account is broken then that can be done online (though it never worked when I tried it, the sim didn't arrive).