milk and fish derivatives.

Found some Sainsbury's own biscuits that are a great protein and fat percentage but they say they have fish and milk derivatives, will these be ok?

I wouldn't give my hedgie them just incase.

I wouldn't give my hedgie them just incase.
I'm sure the fish oil in the cat kibble is okay tho.

Sounds dubious. I had a look on their website, and could only see two dry cat foods of their own brand, Sainsbury's Complete Rabbit, Duck & Chicken and Sainsbury's Complete Senior Chicken, Carrot & Rice. Protein and fat is about right on both, but the first five are a four letter word beginning with C and meaning poo

Cereals - no nutritional value, used as a filler in cheap food
Meat/Animal Derivatives - Parts from an animal, could be bones, could be chicken legs and beaks, but not 'meat' or 'organs'.
Fish & Fish Derivatives - Fish = bad, and fish derivatives are the same as above, namely scales, fins, and so forth.
And item number 5 (and 6 for that matter) is fat (6 is sugar).

In short...junk food. You could replace the Go Cat component with either of them, but with the milk (and milk derivatives), I wouldn't bother, you're saving £~1.60 on about a kilo, which over the course of feeding one hog is less than pennies a day. Either way, it'd be your cheap tat food, and it may not have the same odour control features of Go Cat (which is why we (pretty much) all include a small amount of it in our mixes), in which case, no point to it.