SFX has posted a list of the "10 Most Crucial British SF Novels" here. They define crucial as "the books that pull off the apparently paradoxical trick of defining the genre by revolutionising it". Their list goes as follows:
Frankenstein, Mary Shelley (1818)
The War of the Worlds, HG Wells (1898)
Brave New World, Aldous Huxley (1932)
Nineteen Eighty-Four, George Orwell (1949)
The Day of the Triffids, John Wyndham (1951)
Crash, JG Ballard (1973)
The Hitchhiker's Guide To The Galaxy, Douglas Adams (1979)
Consider Phlebas, Iain M Banks (1987)
Light, M John Harrison (2002)
River of Gods, Ian McDonald (2004)
So, let's see... I've read all of them except The Day of the Triffids and River of Gods - but the latter is on the Olympus Mons that is my TBR pile. I didn't like Brave New World and I no longer think The Hitchhiker's Guide To The Galaxy is very good. As for the rest - yes, they're excellent novels.
But how "crucial" are they?
Well, it's a very... traditional choice of titles. The first five are all novels claimed by the genre, but many non-genre fans don't even consider them science fiction. And while The Hitchhiker's Guide To The Galaxy is unabashedly sf, it's as popular outside the genre as it inside. So it's not until 1987 and Banks's Consider Phlebas that we have a true genre novel, one that was published as science fiction by an author who self-identifies as a science fiction author (when he has that middle M, of course).
I also question the "defining the genre" and "revolutionising" credentials of some of the books. Frankenstein was certainly seminal, as was The War of the Worlds. But Nineteen Eighty-Four is by no means the first dystopia - Zamyatin's We predates it by nearly three decades, for a start. Ballard was one of several writers - the New Wave - who revolutionised the genre, and Crash is an excellent example of that movement's works - but what makes it more "crucial" than, say, one of Moorcock's Jerry Cornelius novels?
Banks's Consider Phlebas was an early New British Space Opera novel, but as a defining movement New British Space Opera didn't really kick off until the publication of Colin Greenland's Take Back Plenty in 1990. As an indication of this, Take Back Plenty won the Arthur C Clarke Award that year; Consider Phlebas wasn't even nominated when it was published. Of course, New British Space Opera later morphed into New Space Opera and is still going strong.
Much as I like and admire Light, I can't quite see what's so defining or revolutionary about it. It's not like it kicked off a slew of fiercely literary space operas. And opinion on it among genre readers is sharply divided. An important book, yes. Just like Harrison's 1975 space opera The Centauri Device. But crucial?
And finally, River of Gods... which I haven't read. And is set in and about India. But unlike British novels such as The Raj Quartet is not about Brits in India. There has not been, as far as I'm aware, any sort of post-colonial movement in science fiction, either started by River of Gods or in which River of Gods squarely belongs. Perhaps there should be.
Certainly SFX's list is a list of books worth reading. But I think my "crucial" list would look a little different.... Like this, in fact:
Frankenstein, Mary Shelley (1818)
The Time Machine, HG Wells (1895)
Last And First Men, Olaf Stapledon (1930)
The Death of Grass, John Christopher (1956)
The Cornelius Quartet, Michael Moorcock (1968 - 1977)
Desolation Road, Ian McDonald (1988)
Take Back Plenty, Colin Greenland (1990)
Fairyland, Paul J McAuley (1995)
Revelation Space, Alastair Reynolds (2000)
Bold As Love, Gwyneth Jones (2001)