Hearts of Iron IV

You can do this using the focus tree. Simply go down the fascist path and dont let king George abdicate. Most if not all of your subjects like India and Canada will leave you as a result. Once you become fascist you can keep following the focus tree to get wargoals against your former countries. I found India to be the easiest to take over as you can use Sri Lanka as a base. To make it even easier you can wait until Bangladesh tries to separate and go to war with India which happens very soon after India breaks away from you.

Ireland doesnt seem to count, and Canada is guaranteed by the USA. Australia and new Zealand and South Africa are other options too but I found India easiest.