I'm not able to determine how long such a loop would need to be, but there are already 3 lines south for some distance (~300m) over Rockingham Road to the Kettering Road bridge. As implied, to the north there's all the space in the world to play with though.
Well, as an example, for a dynamic loop that would enable a freight at 60mph to be overtaken by a passenger at 125mph, with neither being stopped or slowed, and assuming 3 minute headways (which it isn't), and it all working to the second (which it won't), you need a loop that is around 12 miles long.
For a dynamic loop where a passenger train stops at a station, completes station duties, and then accelerates away without hindering a following non stop train, l depends on acceleration rates of the rolling stock, position of the station along the loop, linespeed a, and the station dwell time. However, using:
3 minute headways
125 line speed and loop entry
a station positioned on the loop such that trains enter the loop at linespeed and then immediately start braking
a 1 minute station dwell
accelerating away on the loop...
the non stop train will only overtake the stopping train when it is about half way through its acceleration away from the station call. This means that the stopping train
must be slowed (or delayed with an extended dwell at the station) in order to get the three minute headway behind the fast train. Or the loop needs to continue until the point where either the stopping train stops again where the fast train doesn't, or the two trains take different routes.