Highly recommend Man in seat 61 website (
http://www.seat61.com/) for advice on the London to Thun sector.
Go to the Switzerland page, and look for the Interlaken trains. Thun is a couple of stops before Interlaken (or put another way, one stop after Bern), and all Interlaken trains stop there.
Completely off-topic but my ancestors come from a village not very far from there.