Potentially buying a Swiss Travel Pass plus cheap Eurostar tickets to London joined with low price Prems tickets from Paris to Switzerland could be cheaper if you’re flexible. Fares for Eurostar are on their site as are the French ones on SNCF connect. In Switzerland Eurail is not valid to Grindelwald only Interlaken and then you get a discount. If you want the Glacier Express everyone pays the supplements whatever ticket you have. The only benefit of this train is no changes and a rather nice restaurant car. The scenery is the same from the local train. The Swiss Card covers most public transport in Switzerland Eurail covers only the railway routes on the main network. Look at the Eurail map for details. Swiss Travel Service maps show where the Swiss Pass is valid. Whatever you decide move fast to book the critical Eurostar trains.

There's a link to map of the pass validity in Switzerland on the Switzerland page, under Domestic routes, but it's too small IMO to be of much use. The original of that map can be found at the SBB website. You'll see that the pass only gives 25% discount on the lines south of Interlaken (you don't need to use a travel day on the pass), including to Grindelwald. You can find the regular fares on www.sbb.ch.

Check the Switzerland page, under the "Scenic trains” tab. The Glacier express is quite expensive but you can also take the normal trains (each hour) on the same route. The Bernina express has some reservation-free coaches at the front of the train. The Golden Pass trains have optional reservations, although the new direct trains from Interlaken to Montreux are very busy. But the other ones are OK.

Firstly I would advise a Global pass so you can travel freely in all 33 countries.

I take an alternative view to those who recommend a mixed approach, especially where long distances are included.

Global passes are totally flexible and are not linked to specific trains. Reservations may be lost of course, depending on T&Cs) but with the exception of Eurostar and a few other SNCF/Thalys trains are cheap enough to write off and get new ones. On such a trip as the one you are planning there are many reasons you may want to change your journey, or add extra journeys. Remember you can travel on an unlimited number of trains in a travel day so a local afternoon or evening excursion is easy with a pass at no extra cost.

The cost of extra travel days reduces dramatically as you increase them. A 4 day pass for an adult costs 246 euro (60 euro per travel day) and a 10 day pass costs 400 euro (about 25 euro per extra travel day). I defy anybody to travel freely and flexibly across Europe for 25 euro per day. 

I also never worry about making every single travel day break even, just that the cost of the pass plus an allowance for reservations is budgeted in my holiday.

The ultimate pass would be a continuous pass so you can travel on any or all days of your trip for a fixed price. A 15 day unlimited pass would cost about 400 to 450 euro per person second class or 530 to 590 euro first class.

OOI my last 10 day pass saved about 70% of the full flexible fares on 2 separate trips across Europe.

It’s a bit illogical to start in Paris, then go to London and then come back to Paris. But it might be too late to change that as you have probably made your bookings. But better it start in London, then Paris, then Zürich.

Anyhow, some other concrete advice for your itinerary as it is. Ordinary Eurostar tickets Paris London are quite oké for your travel days in February, There are tickets available for about 60 EUR per person. Which is generally like the cost of a pass day + Eurostar seat reservation. For your return day to Paris, I can find prices of 44 EUR, but only for the late Eurostars. You won’t make it in time for the TGV Lyria. An extra hotel night in Paris will be pricey and nullify your cheap Eurostar offer.

If you book it in one go, London - Zürich, i can find prices of about 130 EUR per person. Add an extra 146 EUR (Swiss rail Pass) + 60 EUR (Montreux Paris) is about = roughly 400 EUR (quick calculation).

If you’lld have started in London, you’ll find a better deal with ordinary tickets and a Swiss pass instead of a Eurail pass I think. But It wouldn’t be as flexible.

If you stick to your route, I would go for a discounted global pass of 7 days (I counted 7 Travel days, no?) and count about 80 EUR extra seat reservations per person for your Eurostar and TGV. It won’t be much less  as with combining the Swiss pass with ordinary tickets, but you’ll have immense flexibility.

 Avoid TGV Lyria (direct TGV France-Switzerland), it’s reservation fee is just extortionate. It’s almost the price for a cheap, single ticket.

Better book an domestic TGV to Mulhouse from Paris, and take a TER (Regional train) to Basel/Bâle, where you have good connection to Zürich. Eurail is the better deal then.

Similar advice counts for all the scenic trains and special expresses in Switzerland. All the routes can be done via ordinary, regional and reservation free trains :)

Last tip: book your Eurostar reservations as soon as you can! They can sell out, lots of people came into trouble for December.

Please read this carefully to get your reservations:

I agree with all of the above but remember you need to buy tickets from Interlaken to Grindelwald with a discount.

We must reiterate move quickly to book Eurostar whatever you decide. Especially be aware that the number of Eurail passholder places are limited and can sell out a long way in advance. Read the links above about how to check availability.

IF money is a constraint-and it seems that case for most kiwi´s (I´ve just met a family here today-about halfway in the tropics), nothing to blame for that, EUR is not that cheap! also consider/look at BUS (coach for/en/) for the trips to/fro Paris, a normal booking can work out even cheaper as only the suppl for the RES by train. PLus that buses also go overnight-on the ferry between GB and FR. The major ones are FLIX and BlaBla. I wonder a bit why a kiwi-which its own snow capped mountains on South Isl would go to the very, very expensive Swiss to look for more of those. THere are also mountain/scenic raillines in Austria and DE=Germany and even Italy.

A bit later in febr are in many countries main pre-spring/snowy holidays than can lay a lot of pressure on such spaces for train and cheap sleep.

Why would anybody coming all that way be looking to save a few euros and waste a huge amount of time catching a bus in Western Europe? 

Eurostar is readily available, takes 2 hrs centre to centre and has space to move around. With a global pass the cost is 30 euro compared with Flixbus/Blabla who operate maybe 3 a day, takes 7+ hours and starts at £37 single. Yes it can be cheaper than E* flexible fares but this is a site predominantly for train/pass users. 

Within Western countries the rail network with a global pass is infinitely better than most express bus services centre to centre, and the high speed network is absolutely no comparison to even those coaches driven by F1 drivers.

Express coaches would only be considered in competition to UK rail prices, and then only if there were no “saver” fares available on the rail service.

I accept that once beyond Budapest the situation is a little different but this post does not require advice for Central Europe.