Advice needed re trains.....

Also, it appears train tickets have not been released yet or am I looking in the wrong place?

At what place are you looking?

For the internal Swiss trips, you can see what the daily schedule is now (use sbb.ch/en), and assume the schedule and fares will be about the same at the time of your trip. Just note that the fares the site shows are assuming you have a half-price card, so its actually double that for the walk-up fare. You'll have to do the math to see if the half-fare card is worth it for you.

Tickets are available at www.sbb.ch/en. Note it is about 8 hours from London to Luzern, but flying might take you that long as well given need to arrive a couple of hours ahead of the flight. Transport to and from airports takes time and money, too. A Swiss pass is probably a good idea but you have to do the math to figure out which one. A pass is also very convenient and may save you money on mountain lifts as well. At the very least, you should get a Swiss Half Fare Card.

Have you considered staying in the Jungfrau Region instead of Interlaken? It is 20 minutes by train up to Lauterbrunnen and a world apart, right in the mountains. Quite charming and great opportunities for mountain journeys. They do winter wandern where you can walk on groomed trails in winter. No equipment required, although we found trekking sticks useful.

Another factor is what are you planning to do when you are in Luzern, Interlaken, Zermatt, and Montreux. If you want to take excursions on local trains and lifts. For some of these, the Swiss Travel Pass only provides a discount.

A 15-day Swiss Travel Pass is 485 CHF, you could probably get by with an 8-Day Swiss Travel Pass Flex for 445 CHF. But 95% of the times, if you have more than 240 CHF of full fare train tickets, the 30-day Half Fare Card for 120 CHF gives the greatest savings. Without going through the numbers, just your point to point tickets will be close to 240 CHF. Since SBB does not allow purchase of domestic tickets until within 2months of the travel date, you can't see the prices if you use your actual dates, just use a date within 2 months of today to see the prices.

Since March 5 is an international ticket to France and most of the journey will be on a TGV. you can buy a nonrefundable ticket now. The connection using the TGV from Lausanne costs 41 CHF if you buy it today (or very soon). Since this is a deep discount ticket, a Swiss Travel Pass or Half Fare Card will not give you any further discount with that kind of ticket. 41 CHF is dirt cheap for a train to Paris, a full fare ticket is 178 CHF. Since this is an international ticket, it is for sale now at www.sbb.ch/en.

Also, it appears train tickets have not been released yet or am I looking in the wrong place?

Train timetables change on the 2nd Sunday in December (9th December 2018). Swiss times have already been published for 2019 and are fixed up to 8th December 2019. they can all be seen on the SBB website: https://www.sbb.ch/en
Same applies for prices. they are fixed and will be the same price for any date in 2019.

If you are buying full price or half price tickets, just by when you get to Switzerland. There are some discount tickets available on some less popular trains about 30 days in advance, but not on all trains.

London to Luzern, you can either fly to Zürich (cut price airlines available) and then train to Luzern, or train London - Paris (cross Paris by RER) - Basel - Luzern.
Look up times and by tickets (tickets as far as Basel only) on either https://loco2.com/ or https://www.trainline.eu/

Thanks to everyone for your replies. I appreciate you taking the time.

The website I was using was loco2.com, via the Man in Seat 61 website.

https://loco2.com/ doesn't sell tickets for Switzerland. Look at the map at the bottom of the first page. They will sell you a ticket from London or Paris to Switzerland (Geneva or Basel) - Loco2 is what I use. But they won't sell you tickets for an Swiss internal trips (Basel/Zürich airport - Luzern - Interlaken - Montreux etc.).
For Swiss tickets look on the SBB website ( https://www.sbb.ch/en/ ), but unless you are buying one of the limited number of cut price tickets, then there is no discount and no advantage in buying in advance.

Is it advantageous to purchase a pass or half pass ahead of arriving? Or should I just buy them the day I arrive?

It should be both faster and cheaper to fly from London to Zurich than to take Eurostar and TGV trains via Paris. See www.skyscanner.com.

A Swiss Travel Pass or Half-Fare Card both work on mountain lifts and the STP also covers some museums (all features only on the counted travel days of STP). [edited] Since your time in Switzerland is more than 8 consecutive days, you'd be looking at relatively expensive versions of the STP, whereas the Half-Fare Card is a minimal up-front cost.