st Pancreas international station--where's the domestic section for day trip trains?

within the same concourse - just look at the departure boards for your train - likely platform 11/12/or 13 but check to make sure

https://www.seat61.com/stations/london-st-pancras.htm#platforms

Pancras - it ain't no body part

Services to the North go from the far end of the terminal, as do Southeastern services to Kent.( Canterbury)

The tracks are on the upper level. If you're looking on Google maps, you're likely only seeing the lower level. Google St. PANCRAS (not Pancreas) station maps, and click on images instead of maps, and you'll likely find what you're looking for.

And here I thought he was the patron saint of digestive fluids.

Oh yes, the sainted bodily part again. Spell checkers have a lot to answer for. Eric, it's St Pancras, no 'e'.

It's all signposted by platform numbers. Just find a monitor when you get out of the tube, and follow the signs.
If you look at the plan here: https://stpancras.com/media/208890/stpancras_map_20170817.pdf trains to Canterbury are from platforms 11-13 at the far end on the right.

St Pancras has actually got 4 sections:
Platforms 1 to 4 - Trains to East Midlands. This is the route for which the station was built, until it was re-built and enlarged for the High Speed line.
Platforms 5 to 10 - Eurostar
Platforms 11 to 13 - High Speed trains to Kent
Platforms A and B - commuter services. These below the main concourse.

The Bakerloo line DOES NOT go to Kings Cross/ST Pancras tube station. You don't say which tube station you will be coming from, but if it is only on the Bakerloo line you will need change somewhere.

See tube map here: http://content.tfl.gov.uk/standard-tube-map.pdf
You have a choice of Piccadilly, Northern (city branch), Victoria, Circle, Metropolitan, Hammersmith and City, but not Bakerloo (brown on map).
When you exit the tube, follow signs to St Pancras, not to Kings Cross which is a different exit.

Thanks to all. You collectively have told me what I want to know, especially about the platforms. Some of the errors on my part were simply I'm juggling several different day trips at the same time.(in terms of planning, I mean) Bakerloo-- I was thinking of my trip to Warwick, via Marylebone. and yes, pancras--what just too busy to notice...

When you go up to the Southeastern High Speed area it is in the far north east corner of the station, as far as possible from the Underground. There are stairs, a couple of escalators and a lift to get you up there so that part is pretty easy.

The High Speed Javelin trains have no First Class or reservations so you can sit anywhere on the train. Seats are padded but not a lot, and quite comfortable. The ride is very smooth and you will be up to speed and back down again for Stratford International station where the most recent London Olympics was held. Then back up to speed for crossing the estuary and onwards. They tend to run to time. Whilst there are toilets and luggage racks there aren't any other amenities. There is nominally free wifi but because a lot of the trip is in tunnels (in order to make the tracks straight so the speed can be high) wifi is at best spotty.

That said the journey is fast so you won't miss what's not there. Get your coffee from one of the many outlets you will pass between the Tube and the train as you walk the length of the station.