Just turn up at the ACO and ask. They enjoy a laugh as much as the next soldier.
And no. You can't just join. All UK SF are recruited from serving and experienced time served soldiers. It's stops idiots trying you see.
If there's one thing I've learnt from being in the Army, it's never ignore a wah. I knew a Major, who got wah'd, made the mistake of ignoring the wah. He wah'd it! Fatal error! 'Cos it turned out all along that the soldier who wah'd him had been wah'ing a lot of other officers who wah'd their wahs.
In the end, we had to disband the SRR. Morale totally destroyed... by wah!