Pages 134/135 of the manual gives information on lost/damaged/replacement keys:
Any key made independently outside of MG
Authorised Repairer Network may not allow your car
to enter READY mode, and may affect the safety of
your car. To obtain a suitable key replacement, it
is recommended that you contact an MG Authorised
Repairer.
Note: The new key cannot be offered to you
immediately because it requires programming to the
vehicle by the MG Authorised Repairer.
Hope that helps.