I think it might have been this: https://www.realtimetrains.co.uk/service/gb-nr:17259/2024-10-15/detailedyes it is the Bedford to Bletchley section but my thought was to wonder whether it was route learning for when Bletchley to Oxford opens.
It's the only light loco that run on tuesday afternoon in that direction according to RTT, and it has 5 minutes pathing between Woburn and Fenny Stratford, and 5 pathing and 2 engineering minutes between Fenny Stratford and Bletchley so it could easily have been held at a signal waiting for the time at Bletchley.
There's a return path a little later in the day and it doesn't appear to have any connection to the EWR project. The Derby RTC - Wembley HS and return paths are both VSTP, so were ad-hoc moves on the day.