The available brands of coconut milk in the UK and elsewhere vary markedly in their thickness and composition. The different thickeners and emulsifiers will have an impact on the consistency of the final dish. Additives like guar gum can have a disastrous effect on a Thai curry as the sauce reduces. As a general guide you should always try to use a brand that contains at least 40% coconut milk and the fewer ingredients listed on the label the better. Although I always try to use brands such as Chaokoh or Aroy-D, far superior in my opinion, these can be difficult to find outside of shops that specialise in Southeast Asian foods or unless you have a local Chinatown. These are also usually priced far more competitively than the brands you will find available in supermarkets.Note that if you use a “light” or “reduced fat” coconut milk you will end up with a soup and not a curry!
UPDATE. March 2022. Aroy-D now being stocked by both Morrisons and Sainsbury’s in the UK.
Other brands commonly seen in supermarkets that I avoid are Thai Taste (it doesn’t) Amoy, Blue Dragon, too many additives. Biona is nice but too thin for curry, great for soups or Laksa though.
Steer clear of supermarket own brands at all costs.They must all be supplied by the same company as they were all consistently foul. Chakoh is available on Amazon and the prices for 6 or more cans including shipping are competitive with supermarket prices. Also, currently Sainsbury’s own brand is not bad at all, but I’m always cautious about own brands as they often switch suppliers as Waitrose did a while back with disastrous results.
Just use half a can to start with, if it has separated then use the top part. You can add the remaining coconut liquid later depending on how thick you want your curry to be.The “traditional” method of heating the milk until it separates from it’s oil and using that to cook the paste is not something I recommend as many people find it hard to achieve consistent results with canned coconut milk. There is more information here, Coconut Milk