Ah ok, I phone I've never used and no manual on the web, but it's an Android phone so should be ok.
It sounds like a service centre number issue to me
Open messages, choose settings ( normally three dots top right) then look at service centre Tap on it delete what's in there regardless of it being right or wrong, then enter +447958879890. Select ok note it must be in that format.
Texts don't live forever if after three days they are not delivered they die
I hope so as if the phone allows uk SMS there is no reason for the phone itself not to allow international SMS. There is one last thing you can try, I wait until the new sim arrives and you've activated it, that to do a factory reset on the phone, good luck