Stu, no problems with long scart, get a reasonably priced quality one and you won't have any problems, I've used 5m scarts many times.

I'd run as many cables as the plasma will take and then some...

You haven't mentionned any aerial cables, or satellite cables. Many tvs in the future may run internal satellite decoders for things like freesat.
An aerial cable will allow you to watch a freeview channel while sky is recording two

Component is probably not necessary since you've got HDMI, but you only get to do this once, so again, I'd run a set just in case.

No Sky+ box has HDMI, you need sky hd for this. You should be able to get a HD box and not pay the £10 for the HD channels, so you have HDMI, but no HD

Finally, if you are planning (or already have) an AV amplifier, most come with switching such that any input (component, s-video, composite, etc...) is converted to HDMI so you theoretically only need run one cable from amp to tv.

Finally (take 2). Run some plastic pipe in the wall along with the cables so that you can draw some other unknown yet kind of cable. Much better than burying them in plaster - see below.