Sounds like youre unlucky, or I’m lucky.
I am consistently making planned* connections between Euston and St Pancras, both ways.
*Planned by me - typically 12-25 minutes between timetabled arrival at one and departure from the other. I font know what the official connection time is.
The issue is that the minimum connection time Euston to St Pancras is 35 mins. My departure from St Pancras is at XX37 For whatevever reason the scheduled connection from Euston is frequently this minimum or plus a few minutes. I reckon for standard class coach on a pendolino to train on the domestic high speed platforms 20 mins doesnt really allow much contingency time. Its not the actual walking distance where you can easily better the time in the middle of the night its the conflicting pedestrian flows.
The scenarios I experience are:
Connecting Northern service to Preston/Crewe/Piccadilly cancelled
Avanti service cancelled.
LNWR trent valley service diverted via Northampton
Whenever the two track railway is in use 10 min standing at red signal for crossover to slows followed by further losses as we catch up the stopping train then the mandatory 5 mins wait outside Euston for the train to leave the platform we are about to arrive at.
The reality is that southbound the service is effectively free with 10% cashback, I can see it wouldnt be if your finally destination was a Thameslink service with trains every 10 mins but the fact is for a destination on the Kent Coast it effectively is.