Why does my 10.8kw shower not blown the 32A breaker in >20 years?

The mcb protects the cable not the appliance ( shower) it’s possible it’s wired in 6mm cable, if so it needs upgrading to a 10 mm which can be put on a 50 amp mcb. It’s very likely that the 32 amp is in-operative. This little lot is required to be RCD protected also. My edit…. You can not apply diversity to a shower you have to apply the current rating of the particular shower. Yours is 46 amp, as you don’t use 240 volt you use 230 volt. That’s why I said a 50 amp MCB.

Easiest way around this would be to down size the shower so you don't have to rewire the shower circuit. Although you have a kW value for an appliance this is a max capacity, these are likely start up currents. Your new shower could use more juice for longer and therefore trip the breaker so to play is safe id down size. 32A x 240V = 7.68kW. Not many Showers out there at the rating.

It might not be tripping due to diversity- your shower might not be running at maximum all the time. However as stated above, the MCB is to protect the cable so it maybe smaller to protect the cable and stop fire hazards. Like everything, it has a life span so probably best to get a professional to check this over for you and ensure the workings of the MCB and cable ratings are correct.

The only reason here in this situation that your circuit breaker doesn't trip all this time because of the type of your MCB. we have to know as an electricians that the rate and the type of the circuit breaker playing the big role of protection. The second think is a special location like a shower must have RCD protection in additional .