In this case I was traveling via a more expensive route rather than just a greater distance than the ticket was for. So whilst the distance was greater, the end station was rhe same.You can excess a smartcard ticket or season ticket, if they have their own PAYG.
The Key allows you to travel beyond the validity of your ticket, as long as the station is in the KeyGo area. You get charged from the last station where your original ticket ends.
I actually returned back drom Motspur Park after 7pm and I would have expected to need an off peak ticket. However, checking on the SWR app suggested otherwise and I tapped put at Guildford without issue.