You see, there's a loop tour that you can do, between the CN trains and the BC coastal ferries. I've hosted my kid's UK friends doing the loop.
Look at a map: you fly into Edmonton (about 8hrs out of Gatwick), train to Jasper National Park, transfer to the Prince Rupert train and stop at my place in McBride.
Back from McBride via Valemount to catch the mainline to Vancouver.
Then all the way up the coast by boat to Prince Rupert. My Dad did it, said it was stunning scenery all the way in winter.
Back on the train, transfer to the mainline at Jasper, off to Edmonton and fly home.
The last kid(?) that did it, he told me that he had a rail pass and could get off/on when he pleased, over a few months.