Numbers kept outside part of the UK, stupid folk (7)

I believe the answer is:

ninnies

'stupid folk' is the definition.
(ninny can mean a stupid person)

'numbers kept outside part of the uk' is the wordplay.
'numbers' becomes 'nines' (nine is a kind of number).
'kept outside' means one lot of letters goes inside another.
'part of the uk' becomes 'ni' (Northern Ireland).
'nines' going around 'ni' is 'NINNIES'.

(Another definition for ninnies that I've seen is " Simpletons".)

I've seen this clue in The Times.

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