Faster M code at execution time for nested if statement

replace your if conditions with the below code

if GetVar("BU") = "UK" then
    if [Cost Centre] >= 17000 and [Cost Centre] < 10826 then "EUR"
    else if [Payment Currency Code] = null then "GBP"
    else [Payment Currency Code]
else if GetVar("BU") = "LM" then
    if [Legal Entity] = "aaaaa" or [Legal Entity] = "bbbbb" then "USD"
    else if [Legal Entity] = "ccccc" or [Legal Entity] = "ddddd" then "GBP"
    else if [Payment Currency Code] = null then "USD"
    else [Payment Currency Code]
else 
    [Payment Currency Code]

in the if using and/or  will evaluate every time but replaced them by a nested if can increase the efficiency.

another tips which is related to the data structure, use the condition wich can filter more rows in the first condition series.