I suspect that this a mistake in the en_GB dictionary, which I don't know how to correct, but the good news is that providing you are using 3.0.2, you are able to completely configure the footer contents.
In the Songs tab of Configure OpenLP you can see the footer configuration on the right. Here you can see the variable that you need to change ${ccli_licence_label}
In the Footer template section all you need to do is add this line at the top
<% ccli_license_label = "CCLI Licence" %>
and you will have the British and therefore correct :-) spelling.
