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It's odd.
Ealing, Hounslow & Hillingdon are possibly a trial Tri-borough,
but when I encountered the Commander with three of his henchwo/men the other week,
(I was outside Ealing Police station taking a picture of the 'This Police Station is now closed notice', when four people approached me, asked what I was doing, and did not accept 'Just leaving' as a valid answer. Turned out they were all coppers of varying rank),
the follow up email proclaimed his BCU (Business Command Unit) was still 'just' Ealing & Hounslow.
Latest cuts in funding from Treasury to the Met have resulted in the MOPAC 'solution' of one Police Station per London borough.
As the Met was also reorganised into Tri-boroughs, it is simple to assume it will become one police station per Tri-borough.