Cannot help with a reference, SSB, but somewhere in the darkest recesses of the brain I recall that 'B' and 'H' at LGW do not 'count' as displaced for wake purposes - although it is always your prerogative to do so.
I think 'B' and 'H' are 300mtrs or so (don't have the charts to hand) and I recall a figure of 760mtrs as the 'definition'.