BA says …
"Your Silver membership opens up access to worldwide priority check-in desks, priority boarding and other services, whatever cabin you're flying in. As a result, you can spend more time in the lounge or on the plane and less time standing in line.
Access to Club World, Club Europe and equivalent oneworld business-class check-in desks
Priority boarding
Priority at Baggage Arrival Services desks
Priority over Blue and Bronze Members on reservation waitlists and airport standby lists"

Priority Security is, IIRC, only for those in the Premium Cabins. So if you're down the back, there's no Fast-Track for you. Not that it often makes a lot of difference anyway!!