As I recently wrote above, I am a great believer in that sort of redundancy/'backup' - which is why I have an electric shower which is virtually never used.
However, as I also said to the OP, if the electric shower is only for that back-up purpose, it doesn't remotely need to be 'the best one can get". I would advise someone wanting such a backup to go for the cheapest one that could be easily installed - and, in the OP's case, that probably means sticking with a 7.5 kW one, so that absolutely no changes to his electrical installation would be required.
Kind Regards, John