Chris, you're right, the market for short fiction is much bigger in the US than here (even allowing for the greater size of the population). And markets for short SF are probably greater than for any other genre (with Fantasy probably in second place), unless you count literary/mainstream as a genre. But in the UK we only have one pro market SF magazine (Interzone). However, it's not a problem these days for UK writers since it's easy enough to submit to US markets. Perhaps because of the universal nature of a lot of SF (and more so, Fantasy), US magazines are happy to publish UK writers if they're good enough.
Interestingly, Jim Hines wrote up his report in the recent handbook of the SFWA. In the very same book was an article by an editor at Tor who said he read a lot of short SF fiction and had quite often contacted an author to ask if they had a novel he could see. Which, despite the report busting the myth that you must sell short fiction before selling a novel, I would think is a pretty good reason in itself for trying to place a few short pieces in good magazines.
If you haven't already explored it, take a look at the wonderful short fiction markets site, Duotrope (
http://www.duotrope.com/).
Terry