XC aren’t currently running the direct Newcastle - South Coast (Southampton / Bournemouth trains) and are sticking to an hourly service towards Bristol and the South West* You’d have to change at Birmingham New Street for Oxford, to get these times you’d have to play with the settings to allow slower trains. A Newcastle to London ticket wouldn’t be valid on this route though so you’d still need separate tickets

*except for a couple of additional services to Birmingham / Banbury