Why does my shower switch keep burning out?
Mira seems to be based in the UK, so I'm assuming a 230V electrical mains.

Just taking the nominal figures for power and voltage gives: 10.8kW / 230V = 46.96A.  If you look at the worst case mains voltage (230V - 10% = 207V), you could actually be drawing over 52A.

So, if your shower is drawing as much power as it's rated for (say if the incoming water is very cold, or it's a cold day and you've got the water temperature turned up), a 45A switch is actually underrated for the job it's doing.  I'd recommend replacing it with a heftier switch.