I have moved to an old Iphone while my new phone is replaced and I get an activation error as below
"SIM not supported - the sim card you currently have installed in this iphone is from a service provider that is not supported under the activation policy that is currently assigned by the activation server"
The SIM card worked fine on my old phone?
Assuming that the iPhone is unlocked for use on any network, you'll probably have to put some cash credit on your account in order to pay for the UK SMS that the iPhone and FaceTime and iMessage activation processes needs to send in the background to allow a SIM from a non Apple approved carrier to work. Ridiculous palaver but that's the way that Apple do it unfortunately.
So try putting €1 cash credit on your account and then switch the phone off and on again and wait a little while and hopefully it will get activated.
This might also be relevant...