What you can do if you're using an Android phone is install Orbot
https://play.google.com/store/apps/details?id=org.torproject.android
and use it in VPN mode for any / all apps that aren't able to connect via giffgaff.
There's also browsers such as Opera mini that use their own routing that get round this bollocks, unsure how that works on iOS and if there is any, but there you go.
Also if you have a VPN (on your router?) that should sort it.
Not cool giffgaff!
Yep, here's the iPhone app and here's the Android app.