A circuit for a 7kW shower could have a 30A fuse. Any larger shower cannot.
However since you will also require an RCD, there is little point in using the 30A fuse, as the cost of an MCB to go with the required RCD is less than £5. There is also the question of what else is in the fuseboard, as many of them have a maximum load of 60A or even less in some cases.
It is highly unlikely you will find any shower less than 7kW. Most are significantly more.
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