I have found an interesting article about Rick Stanton and John Volanthen, providing some infos on their rebreathers:
Thailand Cave Rescue. How did British cave divers gain the skills needed to rescue 12 boys and their coach from a flooded cave when navy seals could not?
www.yay.fish
It is not clear if and how much these units were employed during the rescue in Thailand.
Here the Stanton's home-made rebreather:
And here the modified Castoro ARO unit employed by John Volanthen:
Both units are modified for being able to operate with "mixtures", instead of the basic pure oxygen.
I was expecting to see them in the film, instead I did see almost uniquely standard alu tanks of small size (probably AL80).