If you don't need to go by train from London to High Wycombe, you may prefer to buy a Liverpool-High Wycombe return with a route of "via Banbury", and pay the excess to go via London on your way to High Wycombe.
The ticket should cost £56.25, plus another £11.30 to change the route of the outward journey from "via Banbury" to "via London" (giving a total of £67.55). You should be able to pay this at a ticket office or on the train to London, but you can't do it online or at a ticket machine.
As excess fares are something a member of staff may not do every day, you may need to confirm that you only need to go via London on the way to High Wycombe and how much you expect it to cost. And you may need to show whoever it is your railcard in any case.
If you buy the tickets this way, you can leave High Wycombe at any time after 9:30am. There would be no time restrictions for the London-High Wycombe leg, but you could only use it on the same day as the Liverpool-London one.