Firstly,can I stick plasterboard straight to the wall without bothering
with battons.

Yes, assuming the wall is flat and you use the correct adhesive.

Secondly,can i tile onto this plasterboard.

Yes. However, plasterboard disintegrates when wet. Therefore, you must use a
good quality adhesive and use a full fill technique so that the entire space
behind the tiles is solid adhesive, not just ridges with air gaps that water
can penetrate past, soaking the plasterboard.

I only use adhesives that claim suitability for commercial swimming pools.
B&Q Value adhesive+grout is not for me.

Christian.