I travel frequently between the N-W and Kent. The walk from Euston to St P is NOT 10 minutes platform to platform. If my train is a few minutes late into Euston, I miss the St P connection and my journey takes a half-hour longer. It gets tedious.
There is 15 minutes to get from one to t'other so if VT arrives on time I can just walk it via Phoenix Way, Brill Place and the middle concourse at St P. Using the Victoria line is slightly quicker, IF a train comes immediately. Otherwise, the wait is 5-7 minutes and it's burst a blood vessel and chances are 50:50. Not the way to go.
Through domestic sevices to Stratford or Ebbsfleet would be ideal. There's even a couple of turn-around sidings already in existence in the quarry beyond Ebbsfleet.
It seems a lack of imagination in not contemplating the regional to regional possibilities of HS1 and HS2 connected properly and future proofing to provide for them.
London is an obstruction to my journey. OK, I'm only one but I've been surprised how the Javelins have filled up over the last three years as pax' travel patterns have changed to use the new service. And, there is more pressure to put on further services. BGI!
I also do the journey fairly frequently and if you are using an official nr connection time you are very very unlikely to miss your connection walking down Euston Road. Southbound you are more than likely to be able to catch the service 30 mins earlier from St Pancras.
Service frequency though is likely to kill off any prospect of a through service. 3 trains an hour from Birmingham or Manchester conecting in to 2 an hour to Ashford, with a change is likley to be far more attractive than a through train 3 times a day. This is basically what has killed off every XC through service to the south-east HS2 wont be any different.