This was on the edge of some park land near houses, so it may be a garden plant which has seeded itself.
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Confusing that 'dotted loosestrife' leaves (which are stalked as in the photo above) don't have black dots, but 'yellow loosestrife' leaves (which are stalkless) DO have black dots, according to Rose. The name seems to relate to the glands on the petals.
Lat/Lng: 54.4, -6.5
OS grid ref: NW1212