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I would have said "Purple Top Vervain" would be the common name, but I am sure that iSpot "get recommended" knows best.
That aside, I can't agree this particular ID (not to say that I doubt you, TimP!) without more photos - of the leaves, the stem in close-up, the flower cluster from the side would be helpful, and the overall plant, to show scale, habit, general appearance, etc.
How to take close-ups with cheap phone and hand-lens:
Field Guides for Budding Botanists:
Lat/Lng: 51.3872, 0.5063
OS grid ref: TQ744683