Holy crap! How do they expect anyone to use trains? I can get from Sierre to Paris at a reasonable price - a very reasonable price for a direct TGV that takes 5:20. But that's no use whatsoever, if it then costs THREE TIMES AS MUCH for the Paris to London link, which is only 2:40.
EDIT: Many thanks for the purpletigron/purplecthulhu's advice on getting round Eurostar.
EDIT: OK, I can do Sierre to Paris-Lyon for 113 CHF, if I buy it from the Swiss, and using the halbtax card. That's just under £48. And going via the "I am American" part of the Eurostar site allows me to buy the single/non-flexible fare at $89 each - which is roughly £45. That's compared to the insane £300+ it was trying to do me for originally for the Paris to London leg!
So, Swiss Alps to central London for £93 isn't too bad. I just wish it wasn't such an incredible hassle finding this all out. If I was a PA doing this, and factoring in the cost of my time, it'd be another matter.
EDIT: Ooops, forgot time zone differences. That was 2:40, not 1:40