OK, I looked at your other thread, stating you are going London to Switzerland to Paris and back to London, over 13 days, and leaving in roughly a week from now. So it is too late to change your trip to incorporate my suggestion, but I am going to make it anyway in case others want to do this---with more advance planning.
With the same itinerary and order of travel, but hopefully a few more days, you could fly to Milan after your time in London and take the train straight to Varenna on Lake Como ( on hour from Milano Centrale). Next day you continue north on the train to Tirano (1.5 hours) and transfer there to a train on the Bernina Express route, going as far as Pontresina, St. Moritz, Samedan,,etc.
Note that frequent regional trains ply this route on the same tracks and the same scenery as the "real" Bernina Express. But if you want the panoramic cars and other amenities of the BE, check the schedule carefully as you may need to overnight in Tirano instead of Varenna. Nothing wrong with that other than lack of a lake.