Beetroot, keberoxu. I'm not sure if they're eaten in other countries.
It's a root, boiled then sliced, used in salads. Bright purple in colour.

Just to confuse things, here in Norfolk a lot of sugar beet is grown. It's hauled off by the ton to the sugar beet factories, boiled and crushed then crystallised to produce much of our sugar.

I've thought of some more:-

Dunkirk spirit, British sense of humour, Pantomimes, Punch and Judy, growing leeks and other giant vegetables for gardening competitions, garden sheds, allotments, Book of Common Prayer, windbreaks on beaches, sand castles, country dancing, 'In The Night Garden' (children's TV programme).