The problem with most of Blackbird's suggestion is hat while all great books in their own way, are all representative of a Britain that has not existed for quite some time (unless you believe the Disneyland version of Britain)
Lord of the Rings:
completely changed the face of fantasy. Of course, it was also written as an attempt to give Britain its own mythology and is quintessential British.
Sherlock Holmes is
exceedingly British (like Kipling's cakes
) but I'm not sure how much influence it has? Hmm
Frankenstein? Again though, a Britain that hasn't existed for some time, though very influential.
Maybe 1984? Something by Kipling? The Picture of Dorian Grey?
Really, I don't think you can point to one novel and say yes, this,
this is the book that influenced everything that came after,
this is the book that defines Britain . Not with a Brit novel, not with an American. Books and stories do not exist in a vacuum. They all influence what comes after. (Well okay, all the good ones)