You probably aren't going to get much better than the £50.50 Super Off Peak Return from Seaford to Birmingham International via High Wycombe, valid on any train leaving Marylebone after 09:00, so a potential itinerary would be
06:54 Seaford - London Victoria 08:34
Victoria Line to Oxford Circus
Bakerloo Line to Marylebone
09:40 London Marylebone - Banbury 10:42 (Service continues to Birmingham Moor Street)
10:57 Banbury - Birmingham International 11:37 (Service continues to Manchester Piccadilly)
And return would be
18:14 Birmingham International - Leamington Spa 18:37 (Service continues to Bournemouth)
18:46 Leamington Spa - London Maryleboone 20:10
Bakerloo line to Oxford Circus
Victoria Line to Victoria
21:17 London VIctoria - Lewes 22:26 (Service continues to Eastbourne)
22:30 Lewes - Seaford 22:47