As you've worked out, 'Registered' has been replaced by 'Special Delivery' and 'Recorded Delivery' has been replaced by 'Signed For'.
While 'Special Delivery' items are kept separate from other items, 'Recorded Delivery' mail simply goes in with everything else. That means that, while 'Special Delivery' items get tracked throughout their journey, there is no tracking on 'Signed For' post except for proof of delivery at the end.
In practice many posties don't bother getting a signature. I've had plenty of 'Signed For' items simply pushed through my letterbox. (That might be because my postie knows that, if I'm out, he should simply leave 'Special Delivery' items on my step or in my meter cupboard, rather than taking them back to the sorting office. He probably assumes that I don't want 'Signed For' items returned there either).