Lots of folks towing a Mini and I looked long and hard at getting one myself back in 2016. In fact, I made a sincere offer in a brand new one and the dealer undervalued my trade-in by $5,000. When he wouldn't budge, I moved on and bought my Lincoln MKX.
I know LOTs of folks prove otherwise, but the Mini Owner's Manual and the selling dealer advised against flat towing the manual Mini. In fact, that dealer said if I bought the car to flat tow it that the car would come with a letter denying warranty coverage for the transmission.
Not trying to scare you away. As I said plenty of folks do it. If it's a new model be sure they haven't added some automatic eBrake feature that locks up the car when being towed. I've been told the manual Subarus have added this to their cars and flat towing them is difficult.
Do some research on one of the many Mini forums and check things out.
2017 Winnebago Adventurer 37F
2016 Lincoln MKX Toad