There is 100% definitely a choice of opting into sal sac, 100%.
There has to be, that's the law. So, employers who run sal sac have to run non sal sac schemes as well. And salary sacrifice is a change of employee employment terms, which they have to be able to enter into willingly.
I have run sal sac schemes and had the odd person opt out (there are some circs where it could make you worse off, but that's quite technical, and some people don't like it because they like their whole salary to show on their payslip if they're about to apply for a mortgage, for example). And people on min wage cannot be on sal sac, they have to be on a standard scheme, because it's illegal to take an employees salary below min wage.
But, as I said, it's quite technical and few people seem to understand what it is (and assume it just means any pension where the contribution is taken from salary - which it doesn't).