Floor levelling compound on T&G

Self levelling is usually the preferred option for LV floors. One important thing to make sure that all the boards etc are well fixed, and in particular can move relative to each other at the edges - since that is what could crack the self levelling. (you can also get feathering compounds for a similar purpose - basically where you have a good substrate, but just need to finesse any imperfections or level changes.

With my limited experience of using Karndean, they have a couple of different glues you can use, having used the more "forgiving" slightly re-positional one the the first time, I would actually go for the pros preferred "pressure sensitive" one that fixes firmly in place as soon as you roll it. I found it too easy to shift a fitted tile/plank slightly out of position working round it.

I am using Quickstep tiles that click together so it will be a floating floor. I will have a better idea of the hallway floor when I lift the existing carpet

How thick is the Quickstep? The click together bits on thinner tiles are very flimsy. For a bathroom, I'd go with stick down, anyway.

The Quickstep is 5mm thick. The flooring is already bought so have to go with what I have.

Good luck. :)