But if something is approaching on the Up Fast when the train is at Wellingborough, it's better for the overall traffic flow to keep going up to Kettering.
E.g. 1Y07 St Pancras - Corby departs Wellingborough to the DF at 08:11, but there's 1B16 Lincoln-St Pancras passing Wellingborough P2 at 08:13. 1F12 St P - Sheffield follows on the DF at 08:16. Holding 1Y07 by 2.5 minutes to depart behind 1B16 would cause delay to 1F12 (and 1D13 behind it). You could push 1B16 back but then it risks missing it's slot at St Pancras.
By the time it gets to Kettering South Junction, 1F12 is only 3 minutes behind 1Y07 but it then gets a clear run from Kettering towards Harborough, and 1Y07 is 6 minutes after the previous UF train and 4 minutes ahead of the next one.
This sort of scenario would work in theory, but only if the StP-Corby services arrive, and depart, on time from Wellingborough. Most of the time, the Corby service is running late, which (even at a couple of minutes delay) will cause a negative impact to the Sheffield service by the time the Corby service reaches Kettering South, at the very least. More often than not, even a 2-3 minute delay to the Corby (1Y07 in this instance) will also hold up 1F12 behind it departing Wellingborough - I have seen many a time when the Sheffield service has gone through Wellingborough station slowly and proceeded on a cautionary aspect (due to the Corby service ahead of it).
Such a small delay is common practice with the 360s, at least the 07:39 service (1Y05). I am unsure as to why the 1Y05 is delayed as often as it normally is - I can only assume there is congestion further down the tracks with the GTR services criss-crossing on and off the Fast lines, with GTR's getting in the way of EMR Connect or EMR Intercity services.
I agree that holding back the fast Intercity service will not help, since if the Corby service is delayed, then the Lincoln-London service (or wherever the IC service is coming from, having stopped at Kettering) will be delayed further. An ideal scenario for what you describe is to move the IC service (1B16) forwards by a couple of minutes, so it passes Wellingborough at 08:11 instead of 08:13. That way, the 360 can move across to the Down Slow on departure, so it gets out of the way of 1F12/1D13.
Overall, it seems like there's a problem somewhere in that the MML is creaking under pressure with the number of services it is having to deal with, and it seems like the MML can't really take 6 EMRs per hour with the addition of GTR, unless some serious re-jigging is done. Though as mentioned before, uprating the Slows to 90mph would have allowed the Connect services to be well out of the way of the Intercity services, as the Connects would have use of the Slows to Bedford (then Fasts from Bedford to St Pancras).