Yea, I might have been a little rude, but I was just trying to fit in here.
Lets talk facts, not opinions.
As I have said repeatedly, wire size determines what size fuse you need.
No one appears to have been on the boat, so no one knows what the wire size is.
Standards are great but you don't know if they were adhered too.
You don't know if someone has rewired it at some point.
Putting a larger fuse than was in there COULD lead to a fire.
So the Poster needs to determine what the wire size is to know what size breaker should be there
But I am just an ignorant American that couldn't possible know anything about British electricity.