I have done a lot of research on trains with trying to get the best route to London

So we can fly but don't want to do that

So we will in Zermatt for a few days. We want to take a train from Zermatt to Paris with a 4- 5 hour layover then take train to London. We do not want to spend a night in Paris...ideally, we would like a 6:30 train at Paris De Nord to London
WE are looking at getting 2 tickets. One with sbb to Paris. One from Paris to London
Our budget $450USD for 2 people

I was told tonight (August 31st, 2024) is the last night to book to get a good deal because our travel date is Monday September 30th. We have to get back to London since our flight is from there to home in the US

Can someone please help

If you haven't already, check the Seat61 website for all train related information. Eurostar prices are like flights, the earlier you book, the cheaper it is. I don't know what train types you will use for the Zermatt/Paris sector but if it's a TGV, prices are cheapest when first released, then rise from there. Eurostar also travels from Brussels and Lille, might be worth checking fares and travel times.

Firstly I really appreciate the wish to use trains. Thank you.

I use two tools to track long distance trains, rome2rio and bahn.de (they have an English switch).

R2R is showing a 7 h 40m with one change
bahn is showing a lot of sub 7 hour journeys with more changes

then you have to get across Paris from whichever station you get into to Gare du Nord. The Eurostar bit is easy.

alternatively going straight to London by train
r2r is showing 11 hours E250-500
bahn is shows a similar 11 hours

good luck, book early

The cheapest full fare train tickets Zermatt - London for September 30th cost actually about 320 USD/pax
Tickets for October are somewhat cheaper
https://www.thetrainline.com
The journey takes about 11-12 hrs, either via via Lausanne - Paris Lyon - Paris Nord ov via Basel - Strasbourg - Lille Europe.

But will you not have any Swiss Travel Pass or Swiss half fare card?