....which, knowing this group, will result in a particularly
comprehensive answer

I'm planning to travel on a Friday or Saturday evening from Shepherds
Bush to Bletchley, then returning to Euston and back home on the tube,
later that night.

I've got a 16-25 Railcard, and intend to get a Z1-6 off-peak one day
travelcard, and then an off-peak day return to Bletchley. But should
this ticket be from Harrow & Wealdstone (the last calling point), or
Hatch End (the last station in the zones), or is a 'Boundary Zone 6'
available when travelling out of the zones?

thanks,
- martin