Hello, I am Keith, one of the experts on Just Answer, and pleased to be able to help you with your question.
Could you please expand on 'my bank has a deal one million interest only for five years to one company?' At present your question is a trifle obscure.
Then buy the 15 flats in one company's name.
On reflection you might have to transfer all properties to one company to take advantage of the bank's offer. That would be an expensive exercise unless they were registered land in which case you could use a Land Registry transfer, a relatively cheap method.