Yours will be 7.2 or 8.5 kW depends on the heat exchanger, check the cover should be there somwhere. Alternatively check cable size if you have an access to it. This may be helpful: http://wiki.diyfaq.org.uk/index.php?title=Cables#Cable_Sizes
The bigger the cable the more powerful shower you can have without re-wiring. Triton or Mira are the good ones. Good luck.
A lower one would give noticably worse shower performance. You really need a sparky in to check and advise. There is more to it than just cable size, ie switch ratings, mcb ratings etc. It needs to be done right.
I would suggest that the electrics forum might be a better place to start
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