Wow - that must have been added in the last few days. That will be for just the electrical socket, I guess, since the F56 has everything else.
Retrofit might have been possible if Mini were to offer a kit so dealers could do it - but that doesn't seem likely now that they offer it as an original option.
I bought my bike rack second-hand and it didn't have the electrical control box to make the rack lights work. Since I usually only carry one bike, I just don't bother with the lights. Even though my Roadster had bike rack prep, an extra control box is still needed to do things like make the lights work and disable the rear parking sensors when the rack is connected. So maybe the F56 bike rack will be the same, when it becomes available in the UK - and so maybe around the same £400 price.