There are no elite lines at LHR - only Fastrack which is reserved for Business and First passengers. Access is normally controlled by a sticker plonked on your BP - I don't know if upgrading pax get one or not. Note that there is no Fastrack facility in the Flight Connection facility.

And a small, petty point. It is not a codeshare flight with GB Airways, it is a franchise flight. Expect to find a shiny Airbus in full BA colours, seating etc. The principal differences to a mainline flight will be a GB airways magazine and duty free catalogue, a little logo by the door and slightly better catering.