You need to look at the HMRC customs tariff see https://www.gov.uk/trade-tariff/commodities/1208900000?country=&day=21&month=1&year=2014#import
I think this may be the correct heading
1208 90 00 00 Oil seeds and oleaginous fruits; miscellaneous grains, seeds and fruit; industrial or medicinal plants; straw and fodder 08Flours and meals of oil seeds or oleaginous fruits, other than those of mustard
Items under this heading can be imported duty free.
Don't forget you need to pay VAT. How you handle this will depend on the status of your company e.g. are you VAT registered or not.
You don't say which grain the flour will be made from - this will be important in deciding the tariff code and therefore the import duty.
Happy to answer further questions - and you might like to look at my web site.