Own phone number on giffgaff sim?

You already have a British phone number? If not, it's not going to work. You certainly wouldn't want to port your existing number at home to a British SIM, even if it were possible, since then you'd have to continue service with the GiffGaff SIM with your old phone number. Probably not what you want after your trip...

I think that’s only if you are transferring from another UK mobile service and want to keep your phone number. Like switching from Verizon to t-mobile in the US and keeping your number.

Thank you, Andrew and Katheryne. Indeed I do not have a UK phone number, so that takes care of that!