Jim owns his main residence in London. His employer has seconded him to a post in Glasgow for two years. Jim decides to sell his main residence in London and buy a dwelling in Glasgow which will be his main residence for the duration of his secondment. Although the "chain" breaks down on the sale of his previous main residence in London, his next main residence purchase transaction goes ahead as planned.
At the end of the day that is the effective date of the transaction, Jim owns two dwellings, one in England and one in Scotland and has not replaced his main residence therefore the ADS will apply (irrespective of the fact that he owns no other dwelling in Scotland).