The Police should take seriously a threat of any violence to you, if it is believed by you and preferably heard by another or in a permanent form, they will probably not just accept your word, I would return to them and remind them of their responsibilities where you have proof of a threat of violence to you which you believe.
I do not know what type of agreement you have for your garages, you are correct it is not an Assured Shorthold Tenancy but probably a lease agreement in common law which will require at least 7 days’ notice to quit and provides very little protection to the tenant, the problem will be if the tenant does not move out on the day nominated.