Garlic. I know it's "not recommended" to grow from supermarket ones, but they do grow.
I think courgettes are harvested too small/young for the seeds to be ripe.
We used to grow carrot tops in saucers of water when we were children but you don't get a carrot, just green leaves. Apple pips and citrus pips will grow but you might not get a tree suitable for a small garden.
Doncaster, South Yorkshire. Soil type: sandy, well-drained
Over the last 3 years I have used seeds from reduced tomatoes bought in the supermarket. Last year was the best harvest we have had. No idea what the variety was but it didn't matter as they tasted brilliant and they produced fruits for a very long time.
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In the sticks near Peterborough
If you live in Derbyshire, as I do.