Uk prices are fucking terrible too, I envy all my online pals from the us lol and Look into neem meal as well then, if you using neem oil, as well as all the stuff neem oil contains it's high in nutrients, n,p,k,s,ca all sorts mate..
I use it in my soil mixes at 0.5 cups per cuft, also make teas with it for that quick boost or foliar, bubbling it in water for 24-36 hours to extract the secondary metabolites, add kelp meal to that and you get a bunch more, a full nutrient kick as well as pest control