For future reference, the cheapest way to do this is using advance tickets (either direct or from Edinburgh to London and then from London to Bristol). However, you need to do this well in advance to get the cheapest tickets - it's too late now because the prices have gone up to the point that a walk-up ticket is now the same price.
It looks to me like the £100.40 NOT LONDON Off Peak Return ticket is the best choice now. It's possible to get that fare down a little by using obscure routes and stations, but the journey would take all day.
Get a ticket from Glasgow (priced by Virgin, valid via Edinburgh) to avoid CrossCountry's Off Peak restriction scam.
You could try :
Bristol TM - Manchester Piccadilly
Manchester Piccadilly - Edinburgh
First Transpennine offer 34% discount on advances fares for holders of 16-25 railcard, when booked online.
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