Can you guys get Rocky Mounts bike mounts on your side of the pond? I prefer them to the Thule roof mount stuff. They are less expensive (here in the US at least) and seem to be higher quality. The only downside is if you have Thule or Yakima roof bars you have to carry two keys as RM uses different lock cores from either, but IMHO they are the way to go.
Thule recommends that you remove the roof rack and carriers when not using them, which isn't a really bad idea, but if you don't the hardware in the fork clamps can rust. RM makes their skewers etc. out of stainless steel. When I asked if there was any problem leaving a carrier on the car for a day or so in the rain the email response I got from RM was something along the lines of "no, of course not. Why would there be?" No brainer choice...