EDI to London flights can often be cheaper as there are simply more of them. BA to LHR, LCY and LGW, Flybe to LHR and LCY, U2 to LGW, STN and LTN. FR will
be reducing STN substantially shortly. It is a much more competitive marketplace than GLA to London with a substantially higher number of passengers.
There is no real evidence to suggest that EDI is any cheaper to other Euro destinations. It does have the added bonus of more inbound tourists and business travellers against the outbound passengers so potentially that is why some airlines are dumping GLA in favour of EDI.