cumbrianboy,
You're faking the figures:
Air: 30 min check in, 1 hour flight, no baggage (business travellers which seem to be the aim of the game rarely check in a bag), 15 min transfer to the train (generous at SEN) and 50 mins on the train.
Total time from airport to london terminus: 2hr35 mins
If check-in closes 30 minutes before then, there could be a queue, so need to be there 45 minutes before, mode of transport could get delayed, might turn up late as taxi drivers do, mad panic etc., let's book the taxi for 0530 rather than 0545 ... meanwhile for the train one might be able to walk or take a bus and arrive 5-15 minutes before the train departs.
Hand baggage only ... "But excuse me maam, I have toothpaste and deodorant both of which exceed 100ml in volume" ... "Well you'll have to check them in sir for which we're going to sting you X amount and then you're going to need to wait for you baggage in (not so near) London - Southend.
15 min transfer to the train (generous at SEN)
The train is there waiting for them is it? ... LOL
