On every car with disc brakes there are pads both on the inside and outside of the disc. The calipers squeeze the pads onto the disc. In a perfect world the inner and outer pad and the dics should ear perfectly evenly, they rarely do. The manufacturer will quote a wear limit for the disc, easy enough to measure with a suitable vernier..

If the pads are wearing unevenly on the inside face then check for a sticking piston and guide pins.Though I've found [empirically] that with sticky pins it's is usually the outside pad that wears more. Also check the guides where the back plates fit into the caliper and clean off any crud and corrosion.

Check the thickness of the disc if you have a ridge. As a guide, if there is less than 1.5mm more than the minimum specified thickness then fit new discs and pads (not enough meat left on the discs at this point to last a new set of pads).

Decent quality discs are cheap enough that it's not worth trying to eek out an extra set of pads on worn ones. Check out Premier Factors in Walsall, they have an eBay shop and I got my Mintex pads and discs for my 9-3 Aero from them for under a ton, delivered next day. Focus ones will be much cheaper than that (more common car and smaller discs).