Hi jadeasong, and welcome to MySwissAlps!
>> I wish to know if someone has had this experience and will like to share major experiences of traveling by train from london to switzerland with a british passport. I wish to make a clean and less stressful journey.
I have travelled by train from London to Zürich, but on an Australian passport, not a British one.
The fastest way is via Paris. If you go via Paris, the Eurostar arrives in Paris Nord station, and you have to transfer to a different station in Paris to continue your journey.
That's a bit too much mucking about for me, so I prefer to go via Brussels (station: Brüssel Süd/Midi) instead. Once you arrive in Brussels on the Eurostar, it's a simple matter of changing platforms within the same station to continue the journey to Switzerland. The last time I took the Eurostar from London to Brussels was in early 2012.
You can get connections from Brussels to Switzerland (Basel or Zürich) with only one easy transfer (in Frankfurt am Main, Germany), so it's not an arduous trip, though it is a long one (about 9 hours London-Basel, an extra hour to Zürich).
Passport control is done at London St. Pancras International station, so leave plenty of time for that. Once you arrive in Brussels there are no further formalities to go through.
On the way back, passport control is done by British border staff at Brussels, so you need allow sufficient time to go through those formalities. I think they recommend 30 minutes, but personally I would allow one hour at least. However, with a British passport you will probably get through faster. On an Australian or other passport, it can take longer, as you get a lot of questions about your reason for travel, travel funds etc.
Within Brüssel Süd/Midi station is an area that only Eurostar passengers can enter. Passport control is done within this area, and once you arrive in London, there are no further formalities to undergo.
Depending on the time of year, there may be other routing options.