London Midland is why I asked the question! But Tony is right that on longer journeys splits are often less reliable.
Right,
The best you can do on through advance tickets is £96 (£48.05 each): the 09:00 East Coast from King's Cross on the way out, and 11:50 East Coast from Glasgow Central on the way back should do you (there are a couple of other options). This includes the 10% extra discount from the East Coast website. If you're happy to leave very early, you could get the Virgin Trains 05:39 out of Euston for £49 first class (an extra 95p) - this gets you in at 10:36 and includes a free breakfast! These tickets are probably your best bet.
London Midland is significantly cheaper getting you part way, but takes longer. There's then no obvious cheap way from Crewe. The best I can do with this saves you only £6: advance singles Euston-Crewe and Crewe-Euston (£8 each), then Off-Peak Returns to Preston (£21), Carlisle (£33.40) and Glasgow (£19.80), coming to £90.20. This only involves changing once (at Crewe) and keeps you flexible as they're all regular stops on the route. You can save even more splitting the Preston-Carlisle leg: Oxenholme - Preston to Oxenholme Off Peak Return is £16.20 and Oxenholme - Carlisle Off Peak Return is £13. This reduces flexibility a bit more but brings you down another £4.20, to £86. Whether all of that messing about with different tickets and extra hour journey is worth £10 is probably dependent upon your circumstance!