As I have said, it was Primary school, so it must have been a children's book or a teen fiction book. It was about the future in London, England, where the country has become fishermen because the city is halfway underwater. The protagonist is a young girl (about 12-ish maybe...?) and this girl has a cat that travels along with her on this adventure she goes on.So this girl, she sets off on this boat adventure, just her and her cat, I can't remember the reason for this, but I think it had something to do with a grandmother, or the grandmother is a main influence on the girl.
I think the cover had the face of Big Ben on it as a background centrepiece, but only the actual clock part of it, and it had the effect of it being submerged in water. I'm not 100 percent sure on this, but it does seem to stick in my mind something about this sort of design, so I'm hoping it's for this book.
I can't, unfortunately, remember much else about the book,but it is driving me 'round the bend trying to find it.
Thank you, and I hope this was enough to go by! :)