I really never understood why the Design Packs were withheld in the UK, but as
@Mike.46 says, this decision is entirely down to Audi UK who are responsable for configuring all options and prices for their market. They are the ones that offer and finally sell you the car, not Audi Germany. The same as here in Spain where Audi España are fully responsible (and any other country). They can all pick and choose the options that they allow to be configured in their market (although there is very little reason to withhold most options).
In most other countries, there are no trim levels available for the RS3 - no Carbon or Vorsprung. There is just one trim level and
everything else is configurable as an option. Some options are missing due to parts shortages (TPMS for example) and some are available but will eventually delay your build (sunroofs, blind-spot indicators, phone box etc).
I bought mine with the red Design Pack (all red stitching and alcantara steering wheel) whilst avoiding the Design Pack Plus which is the same but with red vents and red seat bolster accents because I didn't want to go "all out". But these options were fully available to order.
I can understand that the red vents and flashy red bolster accents could be considered "tacky" by the UK team (although I don't fully agree with them), but the standard Design Pack should be standard on all models I think. With either green, grey/silver or just black stitching as a no-cost option. I still can't understand why the standard RS nappa leather has black stitching instead of just grey/silver contrast stitching. It makes very little sense.