Reciprocal travel on EMT does not help a great deal in this case, unless the OP prefers a much longer journey time, which may or may not be appropriate.
Let's investigate the cheapest fares (undiscounted fares):
London - Glasgow: £130.60 (Off-Peak Return priced by VT)
100 miles later
Leicester - Glasgow: a whopping 10p less at £130.50 (again Off-Peak Return priced by VT but is not time-restricted)
65 miles later
Sheffield - Glasgow: a slightly reduced £113.60 (Off-Peak Return priced by XC)
40 miles west later
Manchester - Glasgow: a much better £76.40 (Off-Peak Return priced by VT)
but 40 miles north is much better
Leeds - Glasgow: £54.10 (Anytime Return priced by Northern)
but then there is only a limited number of EMT services to/from Leeds at unsuitable hours going north, so it may be better to pay for a £13.50 Anytime Return between Sheffield and Leeds.
So, I think if the OP can put up with a longer journey time, using reciprocal travel to Sheffield, then buy a ticket to Leeds, and a separate ticket to Glasgow on the Settle & Carlisle route would be the best option.
If time is of essence, then I think the Off-Peak Return from London offers good value.