Oat bran is fine if you don’t have a reaction to it. You need to leave some time between trying the buckwheat and oat bran to judge if you do. As for the buckwheat flour, what about a galette (savoury crepe)? Replace the coconut milk with water in that recipe (until you introduce creamed coconut anyway).
I have listed a website for a UK based company which you might find useful. They sell all types of different foods and though they are probably more expensive rhan holland and barrett their range is a lot wider and its probable that some holland and barrets products contain ingredients that are not wise on this diet.