Can we assume it is a Clubman with a split master cylinder and proportioning valve?
I would suggest the master cylinder probably needs an overhaul and some times the proportioning valves will also need an overhaul but they dont usually affect brake operation to the extent you are advising.
And a drum brake mini will stop very well when ALL parts are working as designed, but I have found over the years that with the quality of parts and the lack of use of my cars that the brakes will deteriorate over 12 to 18 months and usually need the wheel cylinders to be replaced over this time frame.
If you are unsure it would be advisable to take your car to a brake specialist as there is nothing more exciting than entering and intersection and trying to stop with the handbrake because the brake pedal has hit the floor

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1969 Cooper "S"
1967 Mini Deluxe
1973 Clubman Van (the fleet spare)
1978 ex 1275 LS ("Wizard" Eaton Supercharged)

2015 HSV GenF GTS(occasional drive & tow car)
2019 MINI F55 Cooper S