'Have a dekko' was (and is) used mostly in the London area, as are two other phrases with the same 'have a look' meaning - 'have a Captain Cook' and 'have a butchers', which are both rhyming slang rather than foreign imports.

What does a dekko mean?

British slang a look; glance; view (esp in the phrase take a dekko (at))

What does have a decko mean?

1. a peek or a glance. Do you think Rick would take a decko at this for me? verb. 2.