South Africa is gone. India (the jewel in the Crown) is gone. Canada, Australia, and New Zealand look to the USA for defense and trade. What is left?
You may be confusing the Empire (gone) with the Commonwealth — still alive and kicking. SA and India are part of the Commonwealth. A subset of the Commonwealth, the “Commonwealth Realm” comprises nations that recognize Queen Elizabeth II as the reigning constitutional monarch (i.e. Head of State). Canada, Australia, New Zealand, Britain, and a dozen other nations are members of the Realm.
Recently, a “movement” began to create a “Commonwealth Union”, which would be a free-trade bloc, a visa-free travel area (like the EU), have a common foreign policy, etc. This idea hasn’t gone too far (yet); but, it is, as I said, an option for Britain.
Oh, I think it could be put together again -- pace Humpty Dumpty. Not too sure about South Africa, though.