That Merc truck is a badge engineered Nissan Frontier.Small world - wife and I were in the UK for a work/vacation trip in October '21. Had some nice weather.
Even got to see Oxford and Cambridge. I would have been gobsmacked to see a GMT400 over there.
Too bad I missed seeing yours.
I did see this strange Mercedes small truck in Liverpool though. And we do not have them here in the US that I know of... So I had to take a picture of it.
My mum lives in the UK (near Reading) and there's always some cool vehicles to see when I'm there.