The best brand of duvet depends on your requirements and preferences.
If you're prone to allergies and looking for a natural duvet, Three Geese is one of the best on the market. 
If you're looking for a cheaper, high-quality duvet, then IKEA duvets are the right choice.
If you want to buy a high-quality duvet, you can look into some UK brands like Silentnight or Panda.
These duvets have different tog ratings, affecting how warm a duvet will keep you while you are all tucked in.