Hydroponics don't exist in the UK.

Sorry I can't help you with your white powder (maybe you can sell it on some
backstreet and do a runner ? ;-))

KentMarine have just released a whole load of FW Fertilizers. I have just
ordered the Potassium from them. As soon as it arrives, I let you know what
else (apart from Potassium) it contains.

Hope you get help with your powder

Greetings
Mad