I find this sort of discussion comforting, because it's easy to assume that some inherent superiority in British SF will infuse my own writing sitting around thinking about stuff. That aside...
One thing that Steve mentioned in the thread about The Scar applies to other British SF: it's politically engaged. Iain Banks is a good place to start because to some extent it's thanks to him that there's a resurgent UK market for intelligent space opera. (I'm not saying that he invented it, just that it takes at least one author to prove to publishers that there's an appetite for a particular type of story.) Banks has described his Culture novels as a take on fun. galaxy-spanning stories that's an alternative to the Heinleinian Wild West ethos of much American Golden Age SF. He has called the Culture "communist" (at least partly as a wind-up) and, I think, "anarcho-libertarian". (Heinlein and Banks may have different takes on "libertarian".)
In the last 20 years I can think of only one SF novel more explicitly political than China Mieville's Iron Council, and that's Ken McLeod's debut The Star Fraction. Charles Stross restlessly explores the boundaries between what we used to regard as personal and the technology that renders privacy obsolete. Alasdair Reynolds constructs plots from flawed characters who need to have totally selfish goals because the rest of the decaying universe can only survive by looking out for number one: the losers in these struggles are snake food. These British authors dress it up in whizz-bang, but a lot of the foundations are serious.
From an outsider's perspective, there are difficulties in writing like this in the US. In the UK, political debate hasn't (yet: we usually adopt American trends after a few years) become as polarized and personal. As in the US, most people aren't interested in politics -- you certainly can't sell a book on political grounds -- but those who are generally won't be offended by reading a differing point of view. I think there's still plenty of great SF being written in the US but a lot of it seems... safe.
I believe that American writers want to write the best stories they can, but they also want to write the stories that their agents (those who have them) can sell. I believe that American agents want to sell the best stories that their authors can write, but most importantly the ones that the editors will buy. The editors want to buy the best stories that are being written, but the ones that will be distributed. The distributors want to buy the ones that they can sell to the retailers. The retailers want the ones that they can sell to the public. Nobody wants a boycott. Nobody wants to risk a political controversy that might alienate half the market. In other fields you might risk it for the publicity, but controversy doesn't drive sales to SF.
It's really easy to set up these chains where you second-guess what the person above you is second-guessing is acceptable. It leads to stories that are interesting and marketable but not necessarily dynamic and provocative. My impression is that American short stories continue to be more interesting than British ones (on average) because the market is (a) much bigger than for British short fiction and (b) less susceptible to external forces. But British SF authors have recently managed to carve out a niche for exciting novels. Often in hardcover (although this may say more about the rising disposable income of British IT professionals).
How that fits with Charles Stross finding a US publisher before a UK one, I don't know. It's just a silly theory.
