En-GB and en-UK port to the same google.co.uk. I have read multiple forum threads on moz from various users and Moz staff have admitted its a bug their end. No major issue and they’re meant to be migrating UK customers from en-gb to en-uk. I have recently just changed the setting within Moz myself.
How to change from en-GB to en-UK within Moz rank checker
1) Login to Moz
2) Click campaign settings
3) click search tab
4) Swap the option over and click apply. Then let Moz re-grab your rankings
People have commented that you do get a different rank reading between en-GB and en-UK. I have had an independent rank checker report something different also. Due to the way search engines work, there is no 100% accurate reading. Only time will tell how accurate the Moz rank checker is.