Self levelling on hardie backer

[essex123456]

im doing our bathroom, so far I have taken up floor boards, put down green chip board and then fitted hardie backer boards. These have been put down with tile adhesive and then screwed down. I'm now fitting Electric underfloor heating. This only covers part of the floor I have posted and been advised to use self levelling to avoid me getting a different height where there is no underfloor heating.

What self levelling do I use? And can I tile on this with adheiseve when it's dry? I'm putting down travitine tiles down. This is a first floor room.

Thanks