For THIS- a pass is now of no use-as you rightly saw that the quota for passholders are full allready. In fact even getting normal seats on those days-around the busiest of the yr (compare to Thanksgvg in USA) will be a tough job and very expensive. Consider also FLYing or the BUS (Flix or BlaBla)

You can get lower prices for BRU->AMS IF you book at least 7 days ahead via nsinternational or b-europe and NOT on the slightly faster Thalys, but the hourly BeNeLux yellow train.

There are several strike (yes, the jolly workers who lay down their jobs for want of better pay or more colleagues) announced that may also affect some of these trips-esp. in/out of GB.

Here in EUR Xmas usually last 2 days=25+6, and service may be severely reduced then and many many places closed. But 26 also tends to be a very, very heavy travelday.

Unfortunately you are far too late to make a reservation for the Eurostar from London to Paris during the holidays with Eurail. You can try to see if there are any pass holder seats on the London - Lille, but I doubt that.

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

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

From Amsterdam back to London, you can also consider using the ferry from Hoek van Holland:

https://www.seat61.com/trains-and-routes/london-to-amsterdam-by-ferry.htm

You might want to switch your Amsterdam and Paris dates if possible. You can get a ferry from Harwich to Hoek van Holland (Port of Rotterdam), from which you can get easily to Amsterdam (and the rest of Europe). There’s an hourly train from London. That way you can avoid the fully booked or horrendidly expensive Eurostars. You get a discount on the ferry as well with the pass.

Or you can take busses from London (but not included in the pass, nor you would be eligble for a benefit).

Beware: there’s a real winter of discontent. The UK Border force is planning to strike in the Christmas Period, so this could effectively hamper ALL travel in or out the UK from December 23rd until New Year’s Day. Make sure you ensure plenty of time to get through border checks, whatever your final option will be. Be prepared for industrial actions in other fields too - bus drivers, railway employees, health care professionals, road workers, border force, airport personel. It’s gonna be something next weeks in the UK

Ended up snagging flight instead, although the price went up another $50 before I could check out :/

Hotels were already booked so couldn't swap. 

Now just need to figure out a good cheap route from Paris to Brussels on the 26th , I am hoping there's a cheap local train

The ferry back to London sounds like a good idea too I will look into that 

It’s the day after Christmas, so a lot of TGV’s will be full. Usually we recommend to take a TGV to Lille, but reservation costs of 10 or 20 EUR. You might want to save on that because of all the extra costs you already had. From Lille (Flandres) you can get to Tournai or Kortrijk/Courtrai from which you have connections to Brussels with local trains.

Book your TGV from Lille here

https://travel.b-europe.com/Eurail-GE/en/booking-tgv#TravelWish and you need a pass cover number: 

Thalys From Paris Nord is very expensive for pass holders with an extortianate reservation fee and there’s none available for the 26th.

In Lille there’s two stations. Lille Europe and Lille Flandres. All local trains leave from Lille Flandres. It’s only a quick 5 minute walk away.

But you can get to quite easy to Brussels now via these options below and without any reservations, since the new train schedule from 11th december:

There’s also a similar connection at 17:19.

With the last train the intercity (Charleroi to Essen), you can also stay a bit longer in the train to get off in Brussels-Central or North, should any of those stations be closer to your hotel.

Same goes for the connection to Amsterdam. Planners will probably guide you to Thalys, but you should take the reservation free hourly IC-train to Amsterdam. That one leaves from the three major Brussels stations (Midi/Zuid, Central, and North) It’s only half an hour slower.

Thank you for your response, would it be easier to buy tickets at Paris Nord or would they also fluctuate in price

If you have a Eurail pass, you don't need to buy anything extra for the journeys Paris - Brussels @BrendanDB showed.

If you don't have a pass and want to buy a normal ticket, then that should be possible in Paris. You can also buy it online via Belgian railways. Enter Brussel Central as your destination (it doesn't show prices to Zuid/Midi for some reason) and tick the box for "no high speed trains” in the advanced search. Price should not fluctuate.

A normal ticket Brussels - Amsterdam is best bought via NS, 7 days in advance. The ticket office in Belgium charges more.