don't think it's ever simulcast in that sense. We've had BBC America show episodes on the same day the BBC debuted them, but not at the exact same time, due to the time-zone difference. Not only is the US several hours later, but Doctor Who airs here as a prime-time show, whereas I think it's late afternoon/early evening in the UK.
The BBC has announced they're doing a worldwide simulcast
of the anniversary special at 7 pm GMT, 2 pm EST, some way early time in Australia. I don't know if it's going to be a single feed going out from London (so no commercials) or if separate broadcasters will have the program ahead of time so they can format it, say for commercials, as necessary.