I've found Horticultural grit can be different grades/sizes from one supplier to another. For the finer stuff, if you can't find it in a GC, try those outlets which sell Chicken supplies - their Grit is usually pretty good.
I think Carol Klein once said she uses poultry grit because it is cheaper. It also has sharper edges and is alkaline, so it is better at putting off the slugs but no use for ericaceous plants.
Posts
https://www.moleonline.com/westland-potting-grit-20kg-1009084
although I didn't buy it from this outlet
I live in west central Scotland - not where that photo is...
https://www.chepstowgardencentre.co.uk/p/potting-grit-20kg/?gclid=EAIaIQobChMIk7aBlvj89QIVjdxRCh2RbAk0EAQYASABEgJ6rfD_BwE
For the finer stuff, if you can't find it in a GC, try those outlets which sell Chicken supplies - their Grit is usually pretty good.