If the lease is let to A, and A is "A business Ltd", and it is now occupied by "New business Limited" then there is potential for a breach.
If it is let to A, a person, who always operated "A business Limited", which is not contrary to the lease or restricted by agreement, but now operates "New business Limited", where A is the owner and B is a friend staff or partner or owner, that is unlikely to be a sublet or assignment.
As always it depends who it was let to and what the lease says....
I suggest that a conversation " hothouse prejudice" is had with tee landlord pdq.
Based on the information posted, I offer my thoughts.Any action you then take is your liability. While commending individual effort, there is no substitute for a thorough review of documents and facts by paid for professional advisers. More ramblings atleaseholdpropertymanager.blogspot.com