If you were starting from Middlesbrough, one option would be:
A £17.15 (Middlesbrough (MBR)-York (YRK) "not Darlington" off peak return, restriction
TF)
B £72.95 (York (YRK)-Milton Keynes Central (MKC) "not London" off peak return,
2V)
C £80.65 (Milton Keynes Central (MKC)-Plymouth (PLY) "any permitted" super off peak return,
YP)
£8.63 (not sure how they round this) excess for the outbound leg to "via Gloucester" (restriction
2V)
£3.30 x 2 Oyster/Contactless off peak journeys between Croydon and Wimbledon.
D £12.00 (Crewe (CRE)-Chester (CTR) off peak return)
Total £197.98
On 31/7, you'd change at York onto a Crosscountry train. You'd use ticket A from Middlesbrough to York (shown on map YM), B between York and Birmingham (maps BY+ES) and C from Birmingham to Plymouth (maps EB+PB).
On 2/8, you'd use ticket C from Plymouth to Wimbledon (map LE), and Oyster from there to Croydon. You'd probably change at Woking.
On 6/8, you'd use Oyster, then ticket C from Wimbledon to Milton Keynes (maps LE+EN), B from Milton Keynes to Crewe (map MA), and D from Crewe to Chester (map CH). You'd have to catch a train which stops at Milton Keynes.
On 8/8, you'd use ticket D from Chester to Crewe, ticket B from Crewe to York (maps MA+MT), and ticket A from York to Middlesbrough. There are rail replacement buses between Crewe and Manchester, and the simplest option would be to catch one of those to Manchester, and two trains from there, changing in York.
There are ways to reduce the price (such as buying tickets to Northampton rather than Milton Keynes), or the time (such as travelling via Warrington rather than Crewe on 8/8), but it depends on your priorites.
You should check if prices with your actual start station, though - the split at York doesn't always make sense, and there may be other options.