Basically, you're looking for a standard Mk1 coach, but with no passenger accomodation - it's literally a large guards van. It's in blue and grey livery and is faded, obviously, it also has a few pairs of double doors! Shouldn't be too hard to find!
That's not a BG - that's a BSK or something similar, half the coach is passenger accomodation. Looks to a) has asbestos by the boarded up windows, and b) have been a parcels van at some point in it's life from the OPV classification!