Good grief. I think I've tired of trying to follow your wriggling around.I meant this one.I never said it was 35A.To the op don't be misled by this bloke, your cable is rated at a maximum 35Amps and is not suitable for a shower larged than 7kw or 40A fuse/mcb.
Because of the installation method, the 6mm cable will have a maximum rating of about 35A, so you are stuck with the 30A fuse.
You might like to comment on why you changed your advice regarding thg maximum acceptable fuse for a cable with a mnaximum CCC of 35A from 40A to 30A between those two statements ... but I suggest that you don't hold your breathing while waiting for me to do any more wriggle-following.
Kind Reegards, John