Just visit your local / not so local Asda or Tesco that has a pharmacy. Ask for a consultation with the pharmacist and tell him / her what you want. You answer a few questions all annswers should be no.
And collect at the counter. 4 X 100g cost £11.00. I bought a pill cutter and cut each one into 4 so 16 X 25g doses for £11.00.