Koss KSC75 clip on for £17? Good sound, but not as convenient as regular headphones.
Superlux HD681 for £25.
If they must be below £20, then there's Sony MDRZX310, for £15 if you want something more like a traditional headphone.
For extra £9-£10, I'd take the HD681 every time over those ZX310. My dad had some of those cheap Sony's, and while they work, they were not that good to be honest.