Sand from limestone, coral, or related materials will add minerals to the water like calcium and magnesium carbonates. This makes the water too hard for soft water fish, and the carbonates will almost always raise the pH. Fine for rift lake fish, but not for most Loaches.
Silica sand does not raise the mineral level of the water, and is a better choice. I think in the UK this is referred to as Silver Sand. Check your local aquarium supply stores, see what kinds of sand they have.
They probably have some sand/shells/aragonite blend that is intended for marine or hard water tanks. Do not use this.
They will likely also have some kind of silica sand, or quartz sand. This is the one you want. It may come in different grades. Very fine sand may pack together, and reduce the water flow through the sand, leading to poor oxygen levels in the substrate. Sand with mixed size particles can do this, too.
You may have to do a bit of research to figure out equivalents, but here (USA) sand is graded by mesh. 30 mesh is pretty fine, 20 mesh is just a little bit coarser. Either of these are good if you want a fine sand look. For a coarser sand something with a smaller number is coarser.
I have used 30 mesh in aquariums. As long as it is reasonably clean (see the chart, most of the sand passes through just a couple of sieves, not a wide range of particle sizes) it works well.
Here is a chart from a local supplier showing imperial and metric sieve sizes, and some 'mesh' values.
http://www.cemexusa.com/ProductsService ... rading.pdf