It can be purchased on any MINI up to a certain age / mileage at the moment. But, there doesn't seem to be any contractual obligation to hold that offer open, at that price, indefinitely
Bear in mind that TLC XL for the R56 MINI has already gone up from £300 to around £380.
I bought the TLC XL, when our MINI was only 3 months, to make sure I didn't lose out if the offer were to be withdrawn or the price were to go up for the R50/53 as well.