First of all, backup your data (manually). Just your photos, contacts etc. BARE ESSENTIALS.
If you are non jailbroken, hit that Factory Reset (Erase all settings and data) and start again (disable Find my iPhone first though).
If you are jailbroken and on iOS 9.3.3 or lower, first delete tweaks one by one after disabling them properly. Delete and reinstall Cydia Substrate (should delete tweaks first to prevent your phone from bricking).
Then install Cydia Eraser and reset your phone (again, disable Find my iPhone first). Once it resets, manually restore your essential data and rejailbreak if you want.
If it still doesn't work, contact your service provider and tell them to block all messages to UK for the time being.