You need seat reservations on the Eurostar between UK and mainland Europe. On the high speed trains between Brussels and Paris you also need a reservation. Pass holder seats are limited both on the Eurostar from London and on Thalys between Brussels and Paris.

When do you plan to travel?

You can travel reservation free between Brussels and Paris with regional trains but that will take longer time.

On popular departures and during high season the limited number of pass holder seats on Eurostar and Thalys sell out weeks, and sometimes months in advance.

The best place to see the availability of passholder seats and make reservations on the Eurostar and Thalys is 

https://www.b-europe.com/EN/Booking/Pass#TravelWish

Press "later trains" and eventually you will find the next available connection. 

If you have a mobile pass you need to generate a Pass Cover Number in order to make the reservation at b-europe. You do that here in the PCN generator:

https://community.eurail.com/news-and-announcements-39/pass-cover-number-generator-is-live-5653

Selamat siang dari disini, mas. Ya-accept that about all trips di kereta-api listrik that the tipical orang turis wants to make needs RES. IF you only want to make these 2 trips, then a pass is money lost-mahal sekali. Just buy online tiket biasa. Merah: untuk BAS, Flixbus

Selamat jalan.

Appreciate the response. I will be travelling between 6Dec to 15Dec. 

If you are not stopping in Brussels you could always take E* to Lille and then hop on a TGV to Paris for a lower reservation cost, or as Mcadv said. if those are the only trains you plan to travel on you could pay standard fares, which include reservations.

You need to start by checking the availability of seats on the Eurostar. Pass holder seats might not be available on those dates.

May I know if the Eurail Global Pass covers the train between Brussels and Bruges, the IC train? TQ.

Sure it does. No reservations required.

Am flying in to Paris. Planned to buy 4days flexible Global Pass to go to London, Brussels, Bruges and back to Paris. 

There are very few pass holder seats available, only 1st class and in the morning, on the 15th from London to Brussels. 

Hmm, with the really limited pass holder seats and expensive reservations for Eurostar (London-Paris or London - Lille/Brussels/Amsterdam) and Thalys (Paris-Brussels) you might be better off buying ordinary tickets.

It's quite short notice, so prices will be quite high. But it'll be better than buying a pass and not being able to fully use it, because you cannot find free places that you can reserve. Which results in eventually having to pay the full monty finally to get there. Resulting in a lot of wasted money. Anyway, buy your tickets asap. Some first class tickets might be more advantageous than second class in this period. 

Domestic trains in Belgium are quite cheap. Especially in the weekend or if you combine it with a museum ticket of some sorts (sometimes 50% off). You can also take Thalys or Eurostar ticket with an "any Belgian station" option. 

I'm travelling to England for work in that period, but we eventually even had to look for other options as well than Eurostar, lot of demand, even with ordinary tickets... And that was some weeks ago… Cynical if you cannot go to a climate related congres by most environmentally friendly option. 

Eurostar, if you read this, we want/need more trains...

Can i know what you mean by ordinary tickets? Is it the ticket that I can buy from thetrainline.com? For this i do not need any seat reservation? If this is the case, does it mean i can sit any seats which are available? TQ

Thalys and Eurostar don’t work with free seating tickets. You’re strictly bound by the time that’s on your ticket and you get assigned seats. It always has mandatory reservations.

Ordinary tickets you can get from:

https://www.eurostar.com/rw-en (London-Brussels)

https://www.thalys.com/be/en (Brussels-Paris)

or you can book every leg of your trip from https://www.b-europe.com/EN, in combination with the return ticket Brussels-Bruges (I recommend this for you).

Every (domestic) Belgian train you also can get your tickets online here https://www.belgiantrain.be/en, or at the station. Ticket machines and offices are quite plenty in bigger Belgian stations. But you can also get them from the Trainline.com

If you buy your tickets via aforementioned ways, no need to buy a Eurail pass.