Seem to be a number of inaccuracies here.
There is no train from Amsterdam to London, and unlikely there will ever be. This is due to the over-zealous government security requirements (now where have we heard that before) on Eurostar which so inflates the cost of running a station with Eurostar services that they just can't justify expanding beyond the present London/Paris/Brussels setup.
If you go from Amsterdam via Brussels you have all the re-check in palaver onto a second non-integrated rail sevice at Brussels which requires you to turn up more than 30 minutes ahead - they just close the gates at this time. Eurostar service to Brussels has been a financial disappointment to them, so intervals before the next one if you miss it are now down to every 2-3 hours.
For those who speak about travel in Britain at peak weekends with seat reservations, you have presumably never experienced that when things get a bit disrupted the railway companies just say "no time to organise the reservation numbers, sit wherever there's a seat". Except there probably aren't any left by this time. Several hours standing ? Even the worst LCC doesn't do that.
If you get put off onto a bus (as mentioned above), don't expect to have a reserved seat on the train at the other end of the blockage.
All this stuff about quoting the railway timetable duration as the journey time. We don't expect an air journey door-to-door to take the aircraft block time, same is true for the railway. This only really applies to car travel.
Of course if you are going to Truro (on the main line, no airport) rail has an advantage. If you are going to Newquay (not on main line, has an airport) the reverse applies.