Solanum muricatum - Pepino Dulce. A South American native, s. muricatum is grown for its sweet, edible fruits. UNFORTUNATELY the pepinos don't travel well so may be hard to track down in the UK. FORTUNATELY you can grow your own in a greenhouse/sunny conservatory, the taste is said to resemble a mix of honeydew melon and cucumber.
Also on these boards
A mat forming perennial that only grows to about 1.5cms in height and will spread to indeterminate width by taking root at the nodes. This is a ground hugging plant that has with tiny, rounded dark green leaves. Blue/white, star-shaped flowers are borne in abundance from spring to early summer and they almost cover the foliage with flowers. It looks great planted around pavers or stepping-stones. This plant can become invasive, so keep an eye on it. Moist to wet soils with high fertility.
Cucamelons: they're grape-sized watermelons that taste like cucumbers with a tinge of lime. And they're totally easy to grow!