When a phone or tablet is locked, it'll only work with the network that it was bought from. Ofcom banned mobile operators from selling locked phones after December 2021, so if you purchased your device in the UK after this date, you should be good to go.
How can I tell if my device is locked?
If you insert your SMARTY SIM into your device and it works, it means your device is unlocked and you're good to go, woohoo! 💃
If it doesn't work (*sigh* 😞) and you're asked for a SIM Network Unlock PIN or see a screen that displays SIM Not Supported, it means your device is locked to the network it was purchased from.
Any devices purchased from Dec 2021 from networks will already be unlocked. Devices before then may need to be unlocked.