If my uncle transfers American Airlines miles to an account I will create on AAadvantage, can I use these miles to buy a ticket from British airways?
You can use American Airlines AAdvantage miles to buy tickets on a British Airways flight through American Airlines. The flight has to be listed as an available flight for miles redemption. Not all flights are eligible for purchase by redeeming miles regardless of if the flight is on AA or a partner airline. If the flight is eligible, it still must be booked through AA regardless of which airline is operating the flight.
This all can be done through the AA website, mobile app (may not be useable for some international travel), gate agent, or live phone operator (additional fee/cost may apply). Although it may cost more, it may be worth it to use a live operator to find the flight that you want.
Calling a partner airline directly will not work. They can not use nor convert your AA miles.
In my experience, live operators have been able to book miles tickets that were not visible on the app or website. This has been especially true on multi-leg journeys when I wanted a specific routing, time, or operating airline. I have flown on three different airlines for the same journey (Envoy, British Airways, Cathay Pacific). All booked through AA on the same booking and ticket.
You can use your American Airlines miles to fly on a British Airways flight. Just go to search for the flight you want on American's web site or mobile app. It will show both American and alliance partner airlines' flights, and you can choose the one you want.
If you're flying from the US to Europe (or the reverse) you won't see partner flights at the lowest award level (Economy Web Saver; 23K miles). Select a higher award level to see these flights.