It's the fruit of Arum maculatum. They are very common throughout the UK. Maybe you just missed them elsewhere because they were in flower and less visible. In a largish garden they'd be fine but they do spread and in my small one they are tolerated but not encouraged. They have many folk names and an interesting history including the roots being used to make a yellow starch for ruffs and being eaten as 'Portland Island Sago'.

Here is a link that might be useful: Arum maculatum