London-Paris-Interlaken-London

Definitely don't rent a car. It would be very complicated anyway with Brexit. Take advantage of the high-speed trains.

On the other hand could you fly home from Switzerland? That way you don't lose a day crossing France again.

Eurail should be good value as Eurostar tickets tend to be expensive. With the pass you still need to pay a 30€ passholder fare (quota on Eurostar services so book early).

29€ reservations for direct TGVs from Paris to Switzerland but if you hop off before the border the price goes down to 10-20€.

The pass gives you 25-50% discount on Swiss mountain railways such as Jungfraujoch (25%). Boats on Lake Thun and Brienz are fully included!

On Day 2 I'd take the last TGV to Lausanne leaving at around 6pm. Finish with Interlaken.

So double check ticket prices for your dates, read these links and decide:

https://www.seat61.com/how-to-use-a-eurail-pass.htm

https://www.seat61.com/interrail-and-eurail-reservations.htm

Thanks for the info. Very helpful. Is the Eurail pass used (day activated) when I take any of the mountain railways? I am thinking of taking a 4 day global pass but, wondering if the 7 day pass would be required between London-Paris-Swiss-London journey?

A pass day isn't required for a discount. However to get to Grindelwald or Wengen you'll likely use a pass day.

For any day trips in Switzerland a pass day is good value honestly.

Thanks. So assuming I can take a narrow gauge to Grindelwald using a Eurail pass day.

In another question you mentioned: “Interlaken - Paris : do not take the cross-border TGV Lyria (37€ seat reservation), instead take the TGV within France in Mulhouse or Strasbourg (limited 10€, then 20€). Doesn't take much longer, max. 30 min. eurail.com”

Does that mean I will have to take a train from Interlaken to Basel SBB, another from basel SBB to Mulhouse or Strasbourg and then change to TGV from Mulhouse?

You don't have to. If you board the TGV in Basel it will cost you 29€ per person (it was 37€ last year).

However if you cross the border on a regional train and board a TGV in Mulhouse or Strasbourg it'll cost less (10-20€ pp).

Actually to get to London (journey I've done multiple times) I'd recommend doing Strasbourg - Paris. You arrive in Gare de l'Est which is a 10 min walk from Gare du Nord (departure station of Eurostar stations) -> much less of a hassle than crossing Paris by RER or subway.

There are also 2 daily Strasbourg - Lille TGVs avoiding Paris altogether but it'd mean a late arrival in London.