>> The route is called "The Golden Pass" and this one seems to be a scenic route, we could take this train from interlaken. The route is Interlaken - Montreux - Paris - London, the only thing is we don't know if anyone has travelled on this route and would like to hear from people who have.
Sorry, I misunderstood what you were looking for. I thought you meant routes outside Switzerland.
Yes, the Golden Pass route is a very popular route. I've done that trip about 4 times so far, and always enjoyed it. It is wholly within Switzerland, and from Interlaken you would be travelling south to Montreux, which isn't an obvious start for a trip to London ;D It also has a northern section, from Luzern to Interlaken via the Brünig Pass.
You could easily do that trip as a day trip from Interlaken, and spend a bit of time in Montreux before going back to Interlaken. Or if going back to London that way, a few days in Montreux en route would be a great idea.
There is more information here:
>> Also in your experience which Swiss rail pass should we buy, given our itinerary in switzerland looks like this
•1st Leg - Zurich 3 Days (When we arrive and we will be travelling by Train to Interlaken)
•2nd Leg - Stay in Interlaken 7 Days and will visit other places such as Jungfrau etc
•Travel from Interlaken to London
So is that 11 travel days in Switzerland? A Swiss Travel Pass (2nd class) for 15 days would probably be best, as it is only CHF 82 more than an 8-day pass, and you would probably spend at least that on days 9, 10 and 11. There's only one way to be sure though, and that is to do your sums: adding up the fares for all the trips you would be making on those days.
>>•3rd Leg - Returning back to Zurich for 3 days before we fly out
If you are going to be doing day trips from Zürich, a 3-day Swiss Travel Pass might be suitable. If staying in Zürich, a 72-hour ZürichCARD might meet your needs.