The problem re: GM free is that it is increasingly impossible to source GM-free soya unless it's organic. So the choices are basically full-on organic, or something like Smallholder Range, which use other UK-grown non-organic but GM-free cereals. Neither choice is cheap.
Depending on your numbers, it may be cheaper to source the individual straights and other ingredients and mix it yourself, but I think you need a license to mix your own feed. Plus there's doing all the research into making a balanced ration...
And sourcing wheat and other ingredients which are GM-free, and are grown and harvested without noxious chemicals... All of which Smallholder Range are doing for you.
Our choice is to support Smallholder Range and regard the additional expense as a contribution to the future of the planet. :/