If you had a bit more time I'd suggest using the LM's sale to get cheap tickets to get as far as Stafford or Crewe, but you won't have time for this option leaving now-ish.
You're unlikely to get a massive saving on a long-distance journey like this if returning a different day. There
might be some saving to be had if you specifically wanted to travel cross-country by splitting at stations along the route to avoid XC's extortionate fares.
If going via London, likely to be no use at all!
If they're willing to walk from Waterloo to Euston, it could save them 90p each, or if they're willing to use
Southern Only tickets from Portsmouth then it could be a massive £3.50 saving!