75 years ago AIUI light aircraft engines would run on the same stuff as commercial flights.
I think it was even easier than that. AAUI, pre-WWII, most engines ran on automotive grade fuel, since (few) engines required higher than 87 octane at the time. Automotive gasoline was (and is - which would help microlights more than heavier metal) available everywhere. Especially for the pioneers, even if 'more modern' engines were available, staying on mogas seems a sensible choice to keep your options open.
I also think that there was a much lower administrative burden. You could fly the routes shown above whilst never straying away from the British Empire. I'd imagine that a call to the local consul would help with much of the paperwork and, especially as a British citizen and even without much corporate support, you would expect to be assisted.
As we all know, aircraft fly the same whether they are over Basra, Bengalore or Bali, but getting to the bottom of and complying with the ANO in those places is a challenge. And those are just 3 examples out of how many?
It would be lovely if there were a more regular London-Sydney 'race' aimed at amateur participants. The 'race organiser' would assist with the administrative burden and planning and fuel could be procured on a bulk basis to enable interim stops to be used, putting more of the focus of the flights back on the flying itself. And doing that without it becoming a corporate circus... (dreams on)
BTW - If there is such a thing - please post a link!
B.