As far as I can tell, the most suitable ticket is (assuming you're doing this departing 08:15 or later) an
Off-Peak Day Return from St Albans Abbey to Woburn Sands, route "Any Permitted"; it's £21.80 at public rate or £14.40 with a Railcard.
It's valid via Bedford in the outward direction (starting at City) because this is, at 42 miles and 9 chains, within 3 miles of the shortest route wholly by rail. The return journey is valid via Bletchley (to Abbey) as this is the shortest route wholly by rail, at 39 miles and 54 chains.
Might just be worth making the conductor aware of what you're doing when or if they come through at Bedford, so they don't get confused seeing you staying on the train past Woburn Sands!
