The OP wrote, "I have been trying to find a way to thicken chicken gravy and make it a nice dark colour."
Kitchen Bouquet ingredients (according to Google):
caramel, vegetable base (water, carrots, onions, celery, parsnips, turnips, salt, parsley, spices), sodium benzoate and sulfiting agents
Gravy Master ingredients (according to Google):
Caramelized Sugar, Caramel Color, Water, Hydrolyzed Soy and CornProtein, Apple Cider Vinegar, Salt, and Spices (Onion, Celery, Parsley, Garlic).
I understand that most people don't mind. I don't like artificial preservatives in my food and I find that some sulfites give an unpleasant, somewhat bitter flavour to food. I also don't eat soy for medical reasons. So, I thought I would find a homemade way to create something that would darken gravy and shared what I found. I don't know if Kitchen Bouquet and/or Gravy Master are available in Northern Ireland, where the OP is from.