hi, your shower will draw 41.3A at 230V so 45 A mcb will give closer protection in case of fault, depends on cable size & method of instalation whether 50A mcb ok, EG 10mm2 cable enclosed in wall only rated 43A, but if in surface trunking is rated 52A or 'clipped direct' on surface good for 63A, not allowing for volt drop due to length of run!
Easy being sparky eh!
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