As said, this needs to happen after the First World War. But if one is going to stick resolutely to the post-1945 POD, there needs to be a handful of primary objectives:
1) Advancing the country's mastery of science and technology (aerospace, automobiles, nuclear energy, et cetera) to a level above rivals, and then maintain that mastery for long periods of time to allow your industrial powers to be able to establish firm market share;
2) Keep the country's industrial and manufacturing sectors efficient and profitable, and have these companies and sectors be able to maintain a presence in the Commonwealth countries;
3) Smoother and more gradual decolonization, complete with building up the civil services, political systems, economies and societies of these countries so that they thrive after the colonial era, and thus have greater reason to stick with the Commonwealth in a more than name capacity;
4) Make India the ultimate example of 3) if you can, though avoiding or indeed probably slowing independence by 1945 is probably ASB;
5) Make relationships between Britain and as many of its former colonies as mutually beneficial as possible in not only diplomatic and trade benefits but also social ones;
6) Retain as many of the Empire's global trading hubs as possible - Singapore, Hong Kong, Aden, Malta, Cyprus, Gibraltar, et cetera - and make maximum use of them to allow both the Royal Navy and Royal Air Force to use them as bases and to allow them to be global shipping hubs. If it is not possible to retain these as crown colonies than at least make them as close as possible with the Commonwealth.
7) Connected to 6) keep Britain's shipping industries as powerful as possible from the ships themselves to the companies moving them. It could be a huge advantage to have one country or a handful of close countries make the rules for your worldwide shipping networks.
8) As America is going to proudly defend capitalism and the Soviet Union is going to proudly defend socialism, find the middle ground while not alienating the other two. This is difficult, but not impossible.