if you mix your own concrete( C35 mix ) for large shed foundation is this ok with building regs

In short yes, there is no problems where the concrete comes from. But as substructure has stated the sensible money would be spent on ready mix. Just make sure any shuttering is secure before you pour! I would also recommend put some re-enforcing bar in the area being concreted.

its just as cheap getting ready mix mate ,most companies do it forr 100 pouns m3,by the time you buy a tonne ballast 8 bags cement for every tonne labour to mix ,clean up ect probably costs more doing it yourself ,phone up mixamate or master concrete these are multi nationals

A shed however large dosnt have to meet building regs, but no it dosnt matter

Substructure is right.Slogging away mixing by hand is a fools game unless you absolutely cannot get a ready mix.Other than that,building control do not check your concrete only the foundation so wouldnt know where it came from! By hand is acceptable anyway so its up to you.