I consulted BA website to see if they had been brave enough to describe something we can judge but alas no.
I did find this though:-
Fast Track Arrivals is available for:
•customers travelling in First, Club World and Club Europe who hold a non-EU passport
•British Airways Executive Club Gold and Silver members who hold a non-EU passport, plus one guest
•oneworld Emerald and Sapphire members
Does that mean Silver can use it? (Whatever it is)