For the record, my return trip was on
this train. It struggled to hit a ton between the Bedfordshire calls.
If it's in a similar condition to the down fast, some patches are ready. IMX, Meridians can top 125 in between 110 TSRs.
120 near Mill Hill? That's interesting; between StP and a point ~1.25 miles outside Greater London (i.e. somewhere in Borehamwood), the MML isn't signed for anything above 110. If not for the ventilation in Elstree Tunnel (which needed work to allow 110 in the first place) or pathing issues (perhaps), I reckon NR would have raised the fasts to 120/125 up to the curve near the RAF Museum/Grahame Park MPATS.
I'd be interested to know what the fastest TSRs on the classic network ever implemented were. I originally thought they'd equalled/set a record on the MML, but know I'm pondering the possibility of 115/120 TSRs having been done somewhere else in the past...