Man...that sucks. I quickly looked up some UK shops and they love to sell the pricey liquid ferts. Screw Athena, they buy their salts from the same manufacturers mix them slightly different and then sell them at 4x the price since they market to cannabis growers.
I know in the US if I buy dry ferts from mainstream agricultural distributers like, Growmore, ICL, Masterblend, Haifa, Southern AG, JR.Peters and Yara the price for 25 pounds each of base salt, Calnit and epsom would run me about $150 if I did a little shopping.
Maybe some of those products are in the UK, if not I would look to see who distributes to the professional agricultural market and try to source from them. Like I said, it's all the same shit in a bag....just with different labels.