What sort of help are you looking for?
I guess you need to do some research on where you’d like to do the Eurocamp portion of your trip. Once you decide that, the rest of the trip will fall into place around it.
I like Chronotrains. Pick how long you’re willing to travel - say 4 hours - and it will show you where you can reach that time from any point of origin.
https://www.chronotrains.com/en?maxTime=4
Travel is very easy in Switzerland if you base yourself somewhere central - near (ish) to Interlaken is fine - you can reach anywhere you line in Switzerland easily on a day trip.
The only night train I can see being useful to you is Nice to Paris.
Interrail has some suggested itineraries which you can use as a starting point for your planning
https://www.interrail.eu/en/plan-your-trip/trip-ideas/recommended-routes/classic-routes
Hi Ralderton,
I've got the first part of my rail journey almost locked down 
Can I have some advice please
What's is the quickest route traveling from London to Interlaken - specifically finishing in Lauterbrunnen
Ideally we want to complete in one day
Thanks !
So no night in Paris at the start, right?
You can look in planners like DB Navigator to get a rough idea, always useful to split search as well (London - Paris, then Paris - x). Some trains in France and Italy will be missing for July but you can check the current times to get an idea, that's what I do.
All that to say... my itinerary would be:
- Eurostar London - Paris-Nord 08:01 - 11:23
- easy 10 min walk to Gare de l'Est, lunch time
- TGV Paris-Est - Strasbourg 13:52 - 15:39 -> 320 km/h all along!
- TER Strasbourg - Basel SBB 15:51 - 17:08 -> old but comfy regional train, running around every 30 min
- from Basel you've got trains every 30 min to Bern and Interlaken (connections are straightforward)
- Lauterbrunnen at 8pm
If you're fine with an early start (usually on the first travel day, it's no problem with the excitement!) →
- Eurostar London St. Pancras - Paris-Nord 06:01 - 09:19
- easy 10 min walk
- ICE Paris-Nord - Strasbourg 09:53 - 11:41 or 1h later
- lunch in Strasbourg
- same as above, Lauterbrunnen at 6pm
Times might differ a bit on your exact travel date.