Needing to get from Amsterdam to London on April 24...

If there are no seats from Amsterdam, try Rotterdam or Brussels. You can get to those cities quickly and easily from Amsterdam without needing a reservation. 

As a last resort, try Lille, but that would be a pain to get to

You can also take the ferry from Hook of Holland to Harwich, which is slow (but very pleasant overnight), and there’s a discount with your pass. 

For Eurostar on a Friday, that's pretty late to book. Which class is your pass?

From Amsterdam or Rotterdam, there's no availability in the direct Eurostars, not even on 23/4 or 25/4. From Brussels there's no availability on 24/4, but there is some availability on 23/4 and 25/4, especially in 1st class:

You can also use Stenaline from Hoek van Holland. They have a pass discount, but they currently also have a special promo that's cheaper, at €17 each:

For the 14:15 ferry, that's all you'll have to pay. For the 22:00 ferry, you'll also need to book a cabin. Book here:

https://www.stenaline.ie/special-offers/irish-and-north-sea-flash-sale

The Stena Line ferry is really good advice. I’ve enjoyed my journey with them, Your pass is valid as far as Schiedam Centrum. From there you hop on the subway to Hoek van Holland (around 5€, use a contactless card at the barrier). In Harwich the train station is connected to the ferry building. All very efficient. ;)

For more: https://www.seat61.com/trains-and-routes/london-to-amsterdam-by-ferry.htm#amsterdam-to-london-by-day-boat

If you’d really like to stick to trains, check every now and then https://www.b-europe.com/EN/Booking/Pass#Outbound (from Rotterdam and Brussels too). Last-minute seats could open up.