Hi Daniel,
That's a pretty cool skillet, I like it. Yeah it has a little rust showing and needs work, but certainly looks salvageable. Can you get lye (sodium hydroxide) in the UK? If it were mine, like James said, I'd soak it in a lye bath first to remove the crud buildup, then a round of electrolysis to deal with the rust. Lot's of scrubbing with soap and a stainless steel scrubbie as you go, and watch it go from ugly duckling to a swan again. If you can't get lye, some members here skip the lye and just do electrolysis, it will clean both crud and rust, just takes a little longer I think...and scrub as you go to monitor progress. Some even use a lye solution as their electrolyte, but I don't..I prefer to do it separately. Can you remove the handle? Looks like a screw there but not sure. If you can, take it off first and clean separately..if you can't get it off you'll have to suspend it so the handle stays just above water. Everything you need to know can be found in the Cleaning and Restoration board on this forum..if you still have questions, just ask! Good luck with it and let us see the "after" pics. 8-)