Measure does not sum in table

MissBI_21 This looks like a measure totals problem. Very common. See my post about it here: https://community.powerbi.com/t5/DAX-Commands-and-Tips/Dealing-with-Measure-Totals/td-p/63376

Also, this Quick Measure, Measure Totals, The Final Word should get you what you need:
https://community.powerbi.com/t5/Quick-Measures-Gallery/Measure-Totals-The-Final-Word/m-p/547907

MissBI_21 not sure but maybe something like this:

New Measure = 
SUMX ( 
VALUES ( YourTable[Employee] ), 
IF ( CALCULATE ( MAX ( YourTable[Employee] ) ) IN { "X", "Y", "Z" }, 1, 
[Existing UK Days Available Measure] )
)
)

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MissBI_21 you have to use measure instead of 1, I just used that as an example.

New Measure = 
SUMX ( 
VALUES ( YourTable[Employee] ), 
IF ( CALCULATE ( MAX ( YourTable[Employee] ) ) IN { "X", "Y", "Z" }, [Some measure here],
[Existing UK Days Available Measure] )
)
)