If i buy a second box, can i hook it up to my current system to watch it upstairs as well? Or... do i have to pay sky £10 a month so they can make money for doing nothing?

you need a sky card for every box you have.
easiest option is length of satellite grade coax and run extension to bedroom.

just orderd enough to run to three tvs in my office/extension - well garage & shed for less than 20 quid from srewfix.