Thanks for your question.
Sadly, on what you have said, the management company can demand this. This is because the lease is a form of contract between the parties. Therefore you are legally bound by the terms and obligations set out in the lease. The management company would appear to have discretion as to whether it agrees to you paying by 12 monthly installments. Therefore it can change this agreement at any time and demand payment in full.
I am sorry but you cannot object even if you think it is unreasonable. The best you can do is to try to persuade them to continue the arrangement but it is not legally obliged to.
I hope this clarifies the position for you legally.
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