I sure can't say with any degree of knowledge suppliers available in the U.K., but here in the "provinces"
there's many choices. I hesitate to suggest all retouches are created equal. For something as important as a medium (yes folks, I consider retouch a medium) or any art material for that matter, you'll rarely go wrong buying the "most expensive" brand in the store. (I use Pebeo, contradictory to what I've said, because it isn't expensive.) I am an enigma. (Just ask my brother, Steve Sweeney.)