How can I register my sim while in the UK?

I got the message below:

Dear Customer, your SIM registration details need to be updated urgently. Please visit the nearest MTN SIM registration point with your valid NIN to avoid service disruption. We apologise for the inconvenience.

Please note I am currently resident in the UK.
How do I resolve this please?
And if not possible, how to I keep my line?
At the moment the line has been blocked for the reason above.