50A mcb should never trip on a 9.5kW load, it will carry 11.5kW for ever!
Is it just an mcb or is it an rcbo? If so it could be tripping due to earth leakage from failing element.
btw would love to know the cable size to your shower 'cos unless its surface clipped the whole way it must be bigger than a 10mm2!
All other methods of installation would leave the cable unprotected by a 50A mcb, 9.5kW should only need a 40A mcb.
Only other thought is to get the running current checked with a clip on ammeter, if less than 50A then mcb could be faulty, or since mcbs are so inexpensive assume it's duff and get it changed anyway, then if still trips shower fubar'ed,
(Usual comments ref work in bath/shower rooms & consumer units ref 'Part P')