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...
Still good, still elite but not as well trained as the British.
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