Pack it on the bigger bag a dispatch it and it should be fine, although I travel to Brazil and bring it with me on the plane to drink, I just always say it's tea.
I've traveled in and out of the UK with small amounts several times and it's always been fine but they've never found it. Something in a marked pouch is better than loose brown powder. They can swab it for what they like though, it's not going to flag up for anything illegal.