I'd say no, for a number of reasons. First off, 6mm isnt thick enough, as the surface needs to be real rigid, or the tiles will crack and/or come loose. 22mm hardboard is ok, IF the spacing of the sleepers beneath (I think they're called sleepers in English, not sure though) is 30cm or less.
If the spacing is wider, up to around 40cm, you can either screw (and glue: it is important to do both) a layer of 13mm plasterboard on top of the hardboard, or pour a 15m layer of concrete (not sure what the particular material is called in english, perhaps someone else knows, it's similar to the stuff u pour over floor heating cables).
The maximum acceptable spacing is 60cm, and if it is that wide, u need a hardboard, covered by ATLEAST one plasterboard, and preferably concrete as well. In total, the floor, not counting the tiles and adhesive, will be 22mm-50mm thick.
By the way: If it's in a wet room, don't use hardboard directly beaneath the tiles. Even if you tank the surface (it's called tanking in English, right? Applying waterproof membrane... ), which is something you should always do in a wet room, any moisture, which might not come from the room itself, but a leak, or whatever, will make the hardboard swell, thus damaging the tiling.
Please note that I am not familiar with the specific regulations used in Britain. I am, however, an experienced
tiler, and certified as such in Sweden.
Open Question: Is 6mm top quality Hardboard good enough to lay ceramic floor tiles over? 
Re: Open Question: Is 6mm top quality Hardboard good enough to lay ceramic floor tile 
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