Companies in UK are still allowed to opt out, Europe doesn't want us to but we do. You understanding of the regs is exactly correct, the WTR suggests 48 hours max but if the company allows you to, you can opt out. However you do still have to have 11 rest or be compensated for it plus a few other things under the WTR.
It might mean that the company in question does keep all employees to 48 hours or if its a driving job or other industries with their own regs then it might differ.