Who dares wins.
Yuuup afraid so! No. Many members of the SAS are members of the Commonwealth.
A regiment of three battalions. That's around 3000 members. However, much of them are intelligence, support and communications personnel. In total there's maybe a thousand or less SAS soldiers...
You do not 'have' to be British, you just have to be selected as a outstanding soldier. Then pass the hardest training in the world
Still good, still elite but not as well trained as the British.