As Tom says, you cannot pick and choose.
You will be given 3 dates to receive your car.... I imagine those 3 dates are spread across no more than a few weeks.
If you simply want to try to get a weeks delay, or a different date than those offered, but within that time frame, they will contact the scheduling dept for you and ask them, BUT no guarantee.
I did ask this exact question last week and was told that if you want to delay beyond a small period, which in itself is not guaranteed, then you dont go to the back of the queue necessarily, but you simply go into the next batch, which is not scheduled at the moment so NO guarantees on date can be given and you certainly can't chose your month etc etc.
I agree with Tom, it is unlikely the dealer can do much more than keep your car for you, the registering may be out of their hands and even if its in their hands, they too have monthly targets, VAT and what have you.
I doubt most car companies will be much more sympathetic in this regard. You take your GT4 when it arrives I'd say and certainly pay for it.
When I got my Ionic 5, albeit here in Spain, I pretty much had to take it the month it arrived, or certainly pay for it at the very least, which makes sense.... it is not fair to keep it on their books and the dealer in this case had to pay Hyundai themselves.
Anyway, if you were January and wanted to delay to "March" you may be lucky and get some date in March, but you may also be looking at April or later and as has been said, pricing is a whole other story as is road tax.....
). And when I raised this with CS, Scott and B&C, CS got back offering me a cancellation. 
