Getting the surface right for tiling is just as important as the tiling itself. I have had to reject jobs in the past because customers or builders have done the prep work incorrectly, just to save money or lack of knowledge but can lead to disaster.

Chipboard - is not recommended for tiling directly on.

Ply-wood - mimimum 15mm WBP or Marine. 6mm ply is a no-no but ok according to some builders.

Standard Floor boards - i've been asked but no you can't.

Anhydrite screed - Needs abrading first then priming.

Primer - people including builders often make the mistake of using PVA, this is wrong. Acrylic primer must be used at all times, either on walls or floors.

There is a website full of professional tilers 'tilersforums.co.uk, they are very helpful with any queries you may have and including jobs that have gone wrong like above that have been wrongly prepped.

As a rule i don't use any ply, i use a board called 'no more ply' instead, it's stable, won't rot, twist, bend etc and it's only 6mm thick and work's out at around £13-15m2,

or

Tile backer/insulation boards are foam boards coated in a fibre glass mesh for excellent compression strength and sound proofing and again much more stable than any wood/ply etc. These are again around £13-15m2.

Although these boards are more expensive than ply, they are as stated more stable and are only 6mm thick. 

Why take chances with your expensive tiles?