There does seem to be a transatlantic divide, as Pam Peters reports in ‘The Cambridge Guide to English Usage’:
The constructions with feet are commoner in American than British
English, by the evidence of language databases. Numerical expressions
with feet tall outnumber those with foot tall by almost 10:1 in
the Cambridge Corpus of American English, whereas it’s 5:1 in the
British National Corpus. The greater use of the plural unit by
Americans reflects their general preference for formal agreement.
Earlier in the article she describes the singular as ‘a conventional, stripped-down expression typical of conversation or no-nonsense reporting’. Of the plural she says that it ‘elaborates the individual measures to the point of redundancy’.
The use of the singular unit can be regarded as part of British Standard English and it is not restricted to any particular British region.