On 26 Jul there is no availability on direct Amsterdam - London trains.
There is space on several Brussels - London services.
Check and book here https://www.b-europe.com/EN/Booking/Pass using this
Take the hourly IC Direct from Amsterdam (this does not require a supplement, even if it is suggested elsewhere that it does) and be sure to leave enough time, at least 1 hour, for check-in at Brussels Midi.
For the night train between Ljubljana and Budapest :
Luckily looks like it has availability ! Go to this page and add Bérlet (pl. InterRail) as a discount : https://jegy.mav.hu/
Prices are between 14€ for a couchette in a 6-people compartment and 22€ in a 3-people sleeper. If you're 3 you can get it for 3 x 22€ then !
Please note that the train arrives well before midnight in the station, scheduled at 22:47. You'll be allowed to board before midnight to get some sleep but the pass day used will be 17th July in any case.
Eurostar : go via Brussels as advised and do not wait to book. The other alternative would be a daytime or overnight ferry between Hoek van Holland and Harwich (30% discount with the pass).