Chaps, I think this has been mentioned recently, but I'll explain what I know about Special Delivery.
Special Delivery items are sent out in special silver plastic envelopes. Each one has a removable label on it, and each label has a 13-digit reference number on it.
This means that LFC know the reference number (and, presumably, can email it to you) before they give it to Royal Mail.
You can't just go and stuff a Special Delivery package in the post box. It needs to be received and registered at a post office.
If you have a 13-digit reference number (every one I have seen ends in 'GB'), but the Royal Mail website is saying 'Come back later' (or words to that effect), my opinion is that the envelope is still with LFC.
Once it has been posted, you should see an 'out for delivery, we expect to deliver on time... ' type message on the morning that they intend to deliver.
On a positive note, I use Special Delivery quite a lot for my business, and all have been delivered on time.
Fingers crossed for you all - lots of unnecessary stress.