Rusty Cast Iron Skillet - how do I get it looking pristine, and working usefully?

I’ve had my cast iron skillet for almost a year now. At first it was relatively easy to clean - I used Maldon Sea Salt and paper towelling, discarding both salt and paper as they get discoloured, and starting afresh. And then oiling and baking in a hot oven.

Lately, though, I have found it is rusty, and I am wondering if this is because I live in the UK, and maybe damper than usual.