I don't believe there are any laws about EU vs non-EU lines, and I believe it's up to each unit how they organise things... i.e., whether or not they have separate lines at all (NWI doesn't), and how they handle those with ILR, or non-EU citizens travelling with EU spouses.
So - the answer is likely to be different for different airports, and the only way to find out is to check with each airport!