A significant question is how big or small does something have to be to be considered an island? Once you reach 'continent' do you no longer qualify?
But the main thing is probably that Britain and Japan become significant powers, THE world powers. I don't even want to contemplate what is going to happen with China, between knowledge of the future, various competing factions for official rule, Japan dropping it's defense only stance and sticking it's oar in, or other southeast asian nations who have come through entirely intact deciding to expand wildly.
To start the uptimer nations would probably have a fairly 'hands off' attitude towards each other, plenty of fish in the sea for them to be busy dealing with, both assimilation and reorganization with so many missing pieces of global infrastructure to work out.
The second phase would be the saber rattling as various islands either strike out on their own or seek to be absorbed by existing nations. For example, take the USA. Most of it is primitive, but a few islands here and there are VERY high tech, populous cities. Do those cities such as Long Island, to name just one, try to recreate the USA? Do they ask Britain for citizenship in the Empire? Does Puerto Rico become the seat of US government or is it Hawaii?
Or am I being too broad, and by Territory you don't mean just islands but things that are considered 'territories' so there is no piecemeal high tech nations along oceanic boarders? In which case would Hawaii qualify or be excluded because it is technically a State not a Territory but not yet a nation? Does all of Cuba go back or just Cuba outside of Guantanamo Bay etc... The devil is going to be in the details of just what comes back and what doesn't.
I do like the fact that questions like, how potent is the Navy of Madagascar compared to New Zealand suddenly become relevant.