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It's my understanding that Brown knapweed is Centaurea jacea, which is extremely rare or extinct in Britain. Is there a feature which led you to exclude Centaurea nigra, Common Knapweed?
There is a cluster of old records for this species along the Thames Valley so it would be a very interesting record if it was confirmed.
Knapweeds are taxonomically complex. Both Centaurea nigra and Centaurea jacea have diploid and tetraploid cytotypes, and the bigger genetic difference lies between the cytotypes and not the species. So there is some question as to where species boundaries should be drawn.
In theory you can tell C. nigra and C. jacea apart by the shape of the phyllaries, but this photograph is too far out of focus.
There is a third form, Centaurea nemoralis, or slender knapweed. Sell, and the 3rd edn. of Stace, have this as a subspecies of Centaurea debauxii (chalk knapweed). This differs from Centaurea nigra in a number of ways, including the stalk being less swollen below the flower. In my experience it is usually pseudoradiate, but here it occurs as a relict of amenity wild-flower seed mixes, and is not necessarily the same as native populations in the south.
I'd guess that the plant here is Centaurea nigra, but there is insufficient detail visible to make a definite call.
I will try to take a more focused picture but i think it is probably likely to be the more common centaurea nigra
Lat/Lng: 51.437, -0.943
OS grid ref: SU735714