All day returns (Off-Peak if appropriate):
Wigan Stations - Crewe;
Wrexham General - Birmingham (valid via Chester and Crewe and no break of journey restriction);
Birmingham - Leamington Spa;
Leamington Spa - Banbury;
Banbury - Oxford;
Oxford - Basingstoke;
Basingstoke - Southampton Central;
give a total of just under £51.
None of which are any use since the OP wishes to return on a different day
I could not find anything better than an off-peak return from Wigan Stations to London Terminals £49, valid on any Virgin train both ways, and then a CDS from London Terminals to Southampton Ctl route SOUTHERN ONLY £16.75 (the OP did say he was looking for it to be cheap), plus two Zone 1 tube journeys £1.30 each, coming to £68.35. Which is more than the SVR route NOT VIA LONDON at £64.30.
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Ah, one more. There are £6.60 Southern advances on many of the xx32 services off Victoria, knocking £10.15 off the total.