Bruges to London by train

when is the trip? How many people? Kids?

This is the one site for tickets from Bruges or any other city in Belgium to London. You have to change trains transfer in Brussels.
https://www.eurostar.com/us-en/trains

This explains the classes of service on the Eurostar
https://www.seat61.com/London-to-Paris-by-train.htm#Eurostar_classes_explained

I am thinking forget the rail pass You can just buy tickets piecemeal.
However You should outline the order of your trip here so people can see if it is good idea.

Eurostar sells an "Any station Belgium" ticket, which, as the name suggests, covers you from Any station on Belgium, via Brussels and the Eurostar train to London (or vice versa). If you select "Bruges" to London on the Eurostar site it will offer you this ticket.
Yes, I know the correct spelling is "Brugge", but Eurostar only accepts the French version, "Bruges"!!!

"Amsterdam, Brussels, Ghent, etc, is it cheaper to buy a "Belgium Train Pass"" - but, Amsterdam is not in Belgium!
No, just buy individual tickets. In Belgium, tickets are valid for any train at a fixed price.

Eurostar tickets are MUCH cheaper three months in advance. Not changeable or refundable at those prices, however. Look at the travel times; London and Bruges are not "well-connected". Will you spend a few days in Belgium? Use the Search box here for comments on visiting the top cities in the Brussels area.

Edit: Most overlooked facts for this trip:
Allow 30-45 minutes pre-boarding Immigration formalities.
Train goes through France, stops in Lille. Make sure no strike problems.
Brussels stop is not Centraal but is Sud/Midi station. No Bruges problem, but not right by Grand Place.
Bruges rail station not really walking distance to many likely destinations. Local bus right outside.
Daytrips by train in Belgium very easy, but rush hour ticketing can require lining up for human clerk.
American Chip and Pin cards do not work in Belgian Rail ticket vending machines. (Old data. Need newer report.)

We are two adults traveling in Sept
Thanks
Gladys

Gladys:

Use Eurostar to book your ticket from Brussels to London. You'll want to do that as soon as possible, as the tickets will be cheaper that way. It's about a two hour trip, and second class is perfectly fine. The only thing you might want to be careful of here is looking at the travel direction vis a vis your seats. If travelling backwards bothers you, you'll need to pay attention to this. There are travel direction arrows when you make your seat selections (another reason to book early).
https://www.eurostar.com/us-en/trains

Travel inside Belgium use this site:

Here is the website for tickets with Belgium. https://www.belgianrail.be/en/Default.aspx

Pass information: https://www.belgianrail.be/en/travel-tickets/passes-cards/rail-pass.aspx

You'll have to decide if the pass is worth it to you - it might be for 10 trips (5 each).

To Amsterdam, you'll need this site:

https://www.b-europe.com/EN

This is another you'll want to book early for better rates. The Thalys train is fastest.

All of these have phone apps available as well.

Tim:
When I was in Belgium last November, my chip and pin worked fine. The year before that I had one chip and pin that worked and one that didn't.

I purchased a Eurostar ticket London to Brugges several months in advance. Prices are lower if you book months in advance. You need to book a trip at a specific time. My airline (Norwegian Air) notified weeks after booking Eurostar, that the time of my flight out of Florida changed from the morning to later in the day. I had to pay an additional fee to change the Eurostar booking.
If you are leaving from the US, hopefully your airline won't pull a stunt like Norwegian did.

Jodi, thanks for that update on NMBS machines.

Thank you for all your help. We just reserved on the Eurostar following all your great advice.

Be sure, when you take your trip, to leave Bruges with enough time to allow 45-60 minute check in at Brussels. You will be going through Belgium Schengen exit border control and UK entry Border Force control before you board the Eurostar. The tickets and online at the Eurostar website it will usually say minimum 30 minutes or you may be denied boarding, the reality is that it is much better to have nearly double that.

I bet you will have a great time both in Bruges and London.