Southampton uses the UKCAT so if you did poorly on that, it's quite likely that was the issue (although if it was the BM6, they might have different widening participation requirements to the KCL extended course that you didn't meet).
If you do better than your predicted grades you can always reapply in a gap year. In the meantime, it's probably best to focus on doing as well as you can in your studies currently, and then weigh up your options once you have your results. You could select the BMS courses for your firm/insurance choices so you have some option of continuing to uni next year potentially, if you decide against reapplying to medicine.