If it's just a high potash feed then plain Sulphate of Potash will do the trick. Chempack High Potash is prized by plant exhibitionists but quite expensive. I would stick to my Levington's or Tomorite as it still has a little nitrogen and phosphate and well as high potassium.

I saw the chempak on Amazon... £8 for 800g which makes upto 1600 litres?! Seems like a good deal if my bottle of levington was £5?

I would recommend

Jobe’s Organics Vegetable & Tomato Fertilizer 2-5-3 Organic Fast Acting Granular Fertilizer with Biozome,

Get out of dodge when you open the bag because it stinks to high heaven, but your veggies will love it and will give you great produce. It won't stink so much once you work it into the soil. It's available on Amazon.com with customer reviews and almost 5 stars. It works for me. Here is the link:

https://www.amazon.com/Jobes-Organics-Vegetable-Fertilizer-Granular/dp/B002YOJDAS

I don't think you can get Jobs in the UK. I stick to Tomorite and use pure sulphate of potash for a winter feed for fruit bushes.