An SD video signal is only going to get to a certain standard so there's a limit to how much improvement you'll get by spending more.
Keep in mind also that £15 price is heavily discounted because scart is an old connection method and demand for scart leads has fallen through the floor; I paid around £60 when I brought mine (back in the days before HDMI).
I used the QED cable with kit that was reasonably high end at the time (a £500 Denon DVD player and a £1,200 Sony 32" grand vega, then a £1,500 Sony 44" rear projector, then a £2,000 hitachi plasma), and it always gave great results.