Firm and Insurance Choice [ Help needed :) ]

I have received all my offers and in a few months need to make my final decision. I have 4 conditional offers and 1 unconditional ( really low ranked university, Anglia Ruskin). Should I choose the conditional offer from my highest ranked uni which are asking for ABB-BBB (University of Essex) as my firm choice and the unconditional from the really low ranked university as my insurance choice?

Why pick the unconditional at all? Why not pick two conditional offers? Do you think BBB will be a struggle to achieve?

Just in case of circumstances which may affect my grades such as missing a B by one mark, or being ill on the day and not performing to the best of my ability. So having the security of being guaranteed a place in uni no matter what will be of ease and may even help me secure a confident BBB+.