A Travelcard from Milton Keynes would be valid via/from Bedford, albeit at a slightly higher cost than a ticket from Bedford.
Interesting that the MKC Travelcards haven't been routed Watford Junction as the Bletchley ones have to stop people doing specifically that (and it is only the Travelcards, so it isn't just one of those general cases where they stuck a route in because Any Permitted confused people). I'd agree valid but high risk of hassle if inspected on board (which isn't that likely but there could be RPIs out; guards seem never to do tickets on the Marston Vale any more as working the train from anywhere other than the back cab is too slow and results in losing time). If not inspected on board you'd be fine as the Travelcard element would open the barriers at St Pancras. Probably high risk of not getting through the gateline at Bedford, though.
A ticket from Bedford is going to be the way to do it.
Might however be worth seeing if there are any Advances from Euston to MKC; in that case it may be cheaper to do it as singles. These can be very, very cheap, e.g. under £5 if offered.
