I guess the battery you currently have fitted is not tall enough for the battery retaining clamp to press down on the top of the battery and hold it in place.
This is quite a common problem as lots of the battery suppliers list the wrong size (height) battery for the Gen 1 Cooper S.
You could either put a piece of timber underneath the battery and lift the whole think up to the correct height or place a suitable piece of timber between the top of the battery and the retaining clamp.
If the battery is otherwise okay and of sufficient capacity for the MCS
I can't see the point in replacing it yet. By the way, the original BMW batteries seem to survive about 10 years in the MCS
according to the date on the battery of quite a few that have been replaced in the last couple of years in my experience.
Also, be careful when removing the battery clamp bolt from the boot floor.....the retained nut (underneath the boot floor) is often rusted tight onto the clamping bolt and if you go at it too hard you can shear off the bolt leaving the end in the nut thread. This is a pain to drill out, then re-tap the thread to fit a suitable replacement bolt!