No, you can't get Carnet in the UK on the "old" MIB1 units. (it was available in Germany - its almost certainly disabled in software)
I suppose in theory you could flash the unit... if you look carefully at the copyright information you will discover that you unit is actually a linux computer, so in theory you could attempt to hack it but of course its not going to be easy and probably you will break it and probably VW won't let you have another.
Apart from any kind of hack on the hardware/software of the unit, carnet needs a subscription which is bound to be tied to some serial number on the box...
I'm sure it would be a fun project, but I'd suggest getting a second hand unit from a scrappy to play with it...