Tiling on caberfloor P5 chipboard advice needed.

[digitaly]

I was thinking of overboarding with WBP 6mm or if I can find it 3mm

You quoted BS 5385 in your first post & now seem to be completely ignoring it!

12mm is, however, generally considered to be sufficient in most light traffic/load areas (bathrooms). A heavy use/load area such as a kitchen/hallway etc could be another matter. Sorry, 6mm ply is nowhere near thick enough & 3mm is a joke; your expensive tiled floor will be doomed to failure. Only other option I would suggest you consider is a decent tile backer board but, again, I will not use them over crapboard.

my caberfloor it 20mm approx and feels very strong already

Define “feels very strong”; the joist size/pitch/span, floorboard material & thickness is what dictates floor rigidity; tell me what they are & I will tell you if it’s rigid enough!

... and no! I don't take advices from building counter staff.
I prefer forums where information and experience move faster.

Sorry I took this;

I'm going to a building merchant today to see what floor board to buy.

as an indication you were going to ask them what to buy!

I'm also working out of a way to make some access panels where pipes and cables runs (in case for future services) and have them not grout but filled with with some sort of silicon.

Your better off running in what ever services you think you may need so you can connect up to them in the future, any form of access panel in a suspended floor will just invite problems & most likely be a failure point; it will also look naff.

The tiles that I'm going to use for the kitchen are porcelain 600 x 600 (pretty big).

You will need a very flat & rigid floor for those, I fear what your currently proposing will not cope.

The as adhesive I will see in selco what they got to offer and try to match with the one suggested on the sticky post.

If your wise, use only quality trade tiling products of the correct type for your application & tiles; BAL, Webber, Granfix, Mapei (but not the stuff sold in BnQ) etc all available from decent, local trade tile outlets or on-line if you can order enough. Cheap own brand/DIY products are mostly crap & restricting yourself to “what’s on offer” from a Builders Merchant chain is a recipe for disaster unless it’s a quality product, especially in marginal applications such as you have.

Back from some shopping and from local takeway since I don't have a kitchen at the moment LOL

I haven't bought anything as I could not find anything. LOL

My Joist size are 10cm x 6cm in line with some bricks and spaced some at 30cm and some at 40cm. The floor it's not very deep (about 25cm deep) as it was built on the actual extension of the house.

All the copper pipes have just been replaced and tested at 8 bar of pressure (with my air compressor and all of them are close to the walls and insulated (I think I have done a good job there).

the kitchen size it's 3x4 meters (not enormous)... I'm the heaviest in the house 11 stones (LOL) The heaviest item I got in the kitchen would be my american fridge freezer and 8 floor new kitchen units.

P.S.
Restructuring my house it's has been the most fun project I ever done and enjoying every piece of it... Except for the dodgy takeways of course