The tests were joint British and Australian tests on a facility that both nations paid for. At the time they were being done, the plan was for us to develop our own nuclear arsenal as well. It didn’t happen in the end, but it was a joint project (some British officials didn’t like that, but as they needed the facility, they agreed).
I always thought it would be the Cannucks who would have been the primary partners in developing a bomb with the Union Jack on it. They were heavily involved in the Manhattan project with us and the Americans...