Those discs have worn dangerously thin; you've almost been driving on razor blades. I'm guessing someone in the past has changed the pads, but not the discs. False economy on the 1,2; decent aftermarket discs can be had for peanuts. The discs are usually more than half worn by the time of the first pad change; common sennse and a quick calculation will tell you they won't make it to the second without wearing beyond the limit.
Edit: having turned up the brightness on my monitor, there's more material remaining than I first thought (but they're still worn out).
I've seen worse; I once found a completely worn out brake pad (with no friction material whatsoever and half the metal backing worn away) on the ground in a parking bay

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You photos don't show how much material is left on the pads (we need a side-on shot), but since the discs have to go and the pads are quite badly scored by the worn out discs, you'll need to replace them anyway, so it doesn't really matter.
Just checked on Euro; current price for a set of Pagid front pads & discs is £60.43. I'd recommend these; the quality is excellent and the anticorrosion coating on the discs means you don't need to degrease them before fitting, saving you a messy job.
Remember to buy a litre of brake fluid; it's always a good idea to change the fluid after you've done a brake job. Do the clutch as well while you're at it and you may save yourself the need to join the "my clutch isn't working properly and I've got difficulty selecting gears" thread.
Given the amount of wear on those brakes, you will need to push the piston back into the caliper quite a long way - in the absence of the proper tool, a G clamp works quite well on the 500 caliper. I'd recommend clamping the brake flexible hose near the caliper and loosening the bleed screw before doing this, so that you don't push the old fluid back through the lines - any contamination could damage the ABS module, which is machined to very fine tolerances. If you don't, keep an eye on the brake fluid reservoir and be prepared to syphon out some fluid, as it could well overflow when you push the piston back. There's no need to do this if you clamp the brake hose.
Take care not to get any brake fluid on the paintwork; it strips paint (better than some of the new-fangled so-called eco friendly paint strippers you buy in B&Q these days).
If you're in any doubt about anything, come back here and ask; lots of us here have done this loads of times.