Yesterday we took delivery of a Candy white Golf Highline with DSG;..........................
That is the equivalent of a Golf GT in the UK.
Different options to the GTI
I thought so at first too but looking at the UK configuration tool, there are a number of differences in options, not least of which are the engines you are allowed to have in the GT. Maybe it's difficult to compare like with like.
You've lost me 
However my Uk GT has "Highline" on one of the sticker codes.
Isn't that just what they call the computer?
Eh?? But you have just said your golf is called a Golf Highline.
Yes Highline is what they class as the computer but it is also what they class as a trim level abroad.
ie. in UK we get Golf S, SE, Bluemotion, GT and then the GTI / GTD
Abroad it comes in Trendline, Comfortline, Highline and GTI / GTD
That is why your Golf Highline is the equivalent as a UK GT and why my GT has a sticker on it stating "Highline"