Hi @KirstyK yes you can use any of the plant milks but not too sure about kefir. As at least some of the plant milks contain thickeners you would need to experiment with amounts plus water. It certainly does make it more creamy.
Blending is a good choice for new users struggling with the texture and as Bee said – you can play around with milks & water mixes. Treating it like a smoothie and blending in a little frozen fruits as well as water will also give you a thicker and smoother texture.
as @Bee and @Phil_C have suggested you can mix your Huel with whatever suits your taste preference best! and one of the best things about Huel is how customizable it is to your liking. I personally like to add 50/50 water & coconut milk to mine but it’s worth experimenting to find a mixture that suits you.