I am pretty certain ( but check properly) that unpaid rent cannot be taken from the deposit. ( rules are different up here)
Maybe, but that is probably going to make getting another property difficult if your references come back as being in arrears. Ditto future mortgage applications if it ends up on your credit history.
I would contact the agent/landlord. They may just let you go, better to have a happy tenant. A previous tennant in the OH’s old flat left after 9 months as they split up. Made sense rather than them inevitably getting into arrears when one moved out even if they were joint & severally liable.
Although, if they’ve been building that long are they not nearly finished?