It's Foxtail Millet, Feverfew. It's a major component of many birdseeds. It's a tropical annual grass, so it probably rarely becomes a weed in the UK. More of a problem--except for the birds--in the southeastern U.S. :)

Thank you Tug for your quick response. I think it is very attractive so I 'm hoping to grow a plant next year. I will cut off the flowers if necessary to contain it.