You’ve done well getting 12 years out of your electric shower, can only guess you live in a soft water area?
When we moved into our house there was a 7.5kw shower fitted which was just about ok in the summer but in winter the flow was so slow as we had it set on max temp.
Upgraded the cable to 10mm. I lifted the boards and did the first fix, sparky came and connected the triton 9.5kw shower up. The difference in temp and water flow was huge. Only trouble was about a year later it packed up as the heat exchanger had scaled up very badly. Very hard water round these parts. Fitted one of those electronic jobbies that supposed to reduce the scaling which does help a tad. We just accept that we have to sort it out every 18months – 2 years.
EDIT – Have got a basic Gainsborough 9.5kw which was cheaper than Triton and Mira. I’ve also been able to remove the heat exchanger from the older shower and descale with vinegar so will try and refit that when our current one packs up so in theory wont need to buy another shower for a while. In theory…. 🙂