As usual while in service for one issue the Mini dealer always seems to find additional issues. I recently was informed that due to rust issues the subframe on my 2015 Countryman All 4 will need replacing in the next 2 to 3 years. They went on to tell me that they will currently replacing the subframe on a Clubman and indicated it was quite EXPENSIVE - of course for that matter what isn't $EXPENSIVE$ on a Mini.

I have never read nor heard of subframe problems on any Mini model. What actually is the subframe? If the subframe truly needs replacing on a 10 year old car with 100 thousand miles - BYE BYE Onslo : ^ (

 

I may be incorrect, but I believe they are referring to the main structural member of the front of the car. It seems to me like an engine out, suspension off, job.

This link discusses with pictures removing the subframe from an R50, they did it leaving the engine in, but suspension and steering components removed.
MINI Cooper Subframe Removal (R50/R52/R53 2001-2006) | Pelican Parts DIY Maintenance Article
If you look at "figure 3" in the linked page. The front of the engine (passenger side of car) is shown, they mention removing the two bolts from the crush tube. In this image, the crush tube is on the right hand side of the bolts and runs up to the bumper parts. The left hand side of those bolts is what I think they'd be referring to as the subframe.
Maybe from that, you can pull your wheel liner and match the picture to your car and have a better idea of what needs to be done to remove it.

A bing.com search of "r60 subframe removal" brings up several forum links and a couple videos discussing it needing to be done, but I don't see any that are R60 specific.

If you have an independent shop around you that knows Mini's well, a second opinion may be in order.

 

Front subframe just rust a little. The rear ones i have really not seen one rust out on the R60/R61


Rear https://www.realoem.com/bmw/enUS/par...r_S_ALL4&mg=33
https://www.realoem.com/bmw/enUS/sho...diagId=33_1561


Rears on the R50-R53 i have seen rust out.