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... and it does look like ottawensis, in which case it could well be B. ottawensis f. purpurea.
On the other hand, your specimen seems very upright (as opposed to the arching habit of the above) so it could be Berberis thunbergii 'atropurpurea'.
Hard to tell from a photo, sorry!
How to take close-ups with cheap phone and hand-lens:
Field Guides for Budding Botanists:
Lat/Lng: 52.9887, -1.3132
OS grid ref: SK462437