"Take a dekko" (sic) I'd imagine comes easily from the British experience colonizing India. It's a "Hobson-Jobson" word, the Anglo-Indian vocabulary. However, I believe some Romani words also entered UK slang via Romanies & travellers' language. If they are pronouncing it "deekh" (rhymes with 'week'), that sounds like the Romani one, whereas "dekh" (rhymes with 'break') is Hindi-Urdu (Punjabi is dekh or vekh; not sure about Bangla). deekh (look) + ing (English progressive) = deeking/deekin' (looking). That would be my conjecture.

GS: To me, it's more likely that "deekin'" is a corruption of "dicking" which has been British army slang for "observers" (most specifically for people observing army movements on behalf of insurgents/ terrorists/ freedom fighters [select according to politics])for at least 35 years, and which itself originally came from "dekko".