Hi Jamie,
It’s definitely down to the model designation rather than an inability to order extras. The premium and plus pack cars are effectively different models which aren’t listed on the scheme, therefore unavailable to us. You could order any available extras for your base spec car but not increase from that base spec. It happens with others manufacturers too but the Mercedes are more vulnerable to it because of the way they designate the increased specs by changing the model. It happened to me when I recently ordered a Volvo XC40. I wanted an AWD version which isn’t available with the pro pack on the scheme. I could have ordered the lower spec and then ordered all the extra bits via options but it would have been incredibly expensive. In the end I went 2WD which was available with the Pro pack. Understanding that the Mercedes is an awesome car, my advise would be to look at different manufacturers to see if they offer a better spec for your money, especially if you want to combine higher spec with a more powerful engine.