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Only needs to be Euro 4 for a petrol to be ULEZ. Euro 6 for diesel.
A turbo is a turbine driven by exhaust gases to force pressurised air into the engine allowing an engine of a smaller size to produce more power. Good for efficiency, but adds complexity and expense when things go wrong.
The older shape of Berlingo are really tough cars and they use a pretty tried and tested engine design, not so sure on the 1.2 of newer ones though. VW/Skoda quality isn't what it once was and personally I would avoid them simply as they aren't worth the premium prices.
I'd definitely avoid ULEZ friendly diesels, the emissions stuff is just too complicated and unreliable. Unless you're doing lots of miles, I'd go for an older non-turbo petrol and something with 3 proper seats, since an Octavia or Superb will still be tight in the back. Something like this should be pretty bulletproof
https://www.autotrader.co.uk/car-details/202311093808862