I've done this particular journey a few times, and it's quite a civilised way to travel, though it's slower and more expensive than flights (booked on the same date). That said, you can always arrange to have a couple of hours in Paris for lunch if you fancy, which the airlines will never allow.
I usually book through
http://www.tgv-europe.ch/en/
It's not very good at figuring out the connection in Paris, though. The trick is to put in Zürich - Paris, select your trains, and when it gets to the "confirmation" screen, then go to the "add another ticket" bit at the bottom right, then sort out your Paris - London bit. Once you've selected your trains, you can sort out payment and ticket delivery. I don't think you can do the "collect from a machine" option at Zürich, but I have collected the Eurostar tickets at Paris Nord without trouble.
For the uninitiated, you will need to get from Gare de l'Est to Gare du Nord in Paris. They're quite close to each other, and if you are taking it easy, allow 20 minutes to get between them (but if you know the way and are a fast walker, you can do it in less). Eurostar also requires you to check in 30 minutes before your train, but coming back, the Paris-Zürich train is just like any regular train, so I'd recommend allowing about an hour on the London bound direction for the connection, and about half an hour in the reverse direction to be on the safe side.
I've it in both 1st and 2nd class, and would say that, unless you are a larger person who wants the extra seat room, there is no benefit going 1st. Eurostar used to give you a nice meal and free booze in the cheap 1st class fares, but ticket changes last December changed that to some rubbish airline-style snack.
If you get the 07.00 of Zürich HB, you can be in London at about 14.30 (UK time). If you leave London at about 10.30, you can be back in Zürich by at about 19.30. Luggage is usually not a problem (provided you can cary/wheel it between the stations in Paris), though I think bicycles require additional booking of limited spaces, but I've never tried that option myself.
For more details, have a look at
http://www.seat61.com/Switzerland.htm