CSJ have some good foods for around £20/30 for 15kg.

Autarky is £20 and the only filler is maize (think it's the 3rd ingredient), i was thinking of putting Millie on this to mix with Wainwright's trays (she refuses to eat without meat -_-).

Skinners - gave my dog wind though.

If you're after cheaper than applaws there's loads.
Fish4dogs i don't know if will work out cheaper but i'm a huge fan.
WW kibble
JWB (careful as they're slowly changing ingredients so i'm wary at the moment)
Arden Grange
Barking heads

Burns do a pork and potato too

Most of them will send samples if you email