CWG horse and pony cubes is £6.80 before the discount and Bury Feeds do Hilight at £5.75 (last time I looked) this is ok I used to feed it when I lived in Hampshire and it cost £4 a bag then!
Bailey's Hilight Horse and Pony Mix at £6.25/20kg and Spiller's Pegasus Value Mix at £6.25/20kg. And, there's also Bailey's Economy Mix Plus at £7.47/20Kg and Badminton All-in-One Museli at £7.80/20kg. And that's buying it online from here Millbryhill.
And I get it cheaper still, when buy it locally.
£8+ does seem very exensive to me for a basic shop's own mix.
Maybe you should print this off and show it to them