Big brake kit for f54 Cooper S with 17" wheels
 

Am I correct to assume that I will need to upgrade to 18" wheels/tires to fit JCW or other aftermarket brakes?

Or could this be done by just getting spacers to further offset the 17" wheels to clear the larger calipers. Or are the replacement rotors too large? Please help me understand this issue, as the standard brakes on the Clubman S don't seem to be that powerful and I didn't want to dish out another $5k+ for the JCW version, when I can get essentially the same with bigger brakes and a Dinan Elite box

You can run 17, 18 or 19" wheels with the BBK. It's the offset/profile that will either clear the Calipers or not. If you don't want to shell out the money for the heavy OEM style wheels there are other manufacturers like NM Engineering that can supply you with a much lighter and IMO better looking wheel that will clear the calipers.

or spacers......
 

or you can run spacers. I'm running 20mm all around with my BBK. ECS Tuning has some very nice quality spacers and lugs.

Good luck

Hoopty

Do I need a larger master cylinder with the BBK? Anything else?

No. I would recommend braided stainless brake lines though.

I have looked at several tuner sites and can't find any brake line upgrade kits for the F54 Clubman S.

Does anyone here have a suggestion about where to look for stainless braided brake lines for the Clubbie?

Have you checked NM Engineering? That's where I got mine for the F56.

might check out goodridge, they have a wide variety of lines