I usually find that the long cooking, plenty of tomatoes and the beans themselves thicken the con carne quite well.
I have added some chocolate, which to my surprise, went really well. I have added sugar (when not using chocolate) to sweeten and take the acidity away from the tomatoes but don't add too much (you shouldn't be able to tell it's there)
I`ve always put a teaspoon of sugar in (assuming making enough for 4). If you simmer it for at least 1/2 hr you shouldn`t need thickening as the tomato juice will reduce itself (I put at least one 400g can in, sometimes 2 & perhaps some tomato puree). As with curries, the heat from the chillis needs to properly permeate the ingredients which requires time.
You can also use a product called Mole (the e has an accent over it, but can't do it on my keyboard!). This contains chocolate and spices and ground corn and is a classic 'finishing' ingredient in a lot of Mexican recipes. Don't know where you can get it in England, but I'm sure some clever person on here will be able to tell you - I got mine from my sister who brought it back from Mexico for me.