The super off peak return is a pretty good deal.
I've found a workable itinerary via Crewe for £70.95:
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Off-Peak Return (inc HST) Ashford-London, 07:13-07:50, £24.05
Walk St Pancras>Euston
Single Advance, London Euston-Crewe, 08:10-09:48, £7.25
Single Advance, Crewe-Glasgow, 10:09-13:17, £20.15
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Single Advance, Glasgow-Crewe, 14:00-16:38, £14.20
Single Advance, Crewe-London, 17:33-20:49, £5.30
Walk Euston>St Pancras
Return leg, London-Ashford, 21:12-21:50
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You may be able to go ever so slightly cheaper if you're willing to do longer journeys (eg 3 hours both ways between London and Crewe rather than just one way) or travel very early/late.