Free Traffic Legal Advice

The legal aspect requires YOU (not the P.O.) to provide the details and documents. It's thus up to YOU to demonstrate what you have attempted. It looks like you've sent it 'signed for' (why?) at the last minute (why?)? It may be that address is a mailbox so there's no-one there to sign for it so it will continually bounce back. Given they're potentially preparing to issue procedings, I'd suggest you a) get the letter returned to you (as it contains your licence/ did they ask for that?), b) resend immediately if time allows by 1st Class post whilst getting a delivery receipt and contact the FPO and explain the situation.

If you drag your heels though, it may not end well.

The legal aspect requires YOU (not the P.O.) to provide the details and documents. It's thus up to YOU to demonstrate what you have attempted.

Twaddle. The requirement is that the information is provided - which, unless the Interpretation Act is disapplied, means delivered.
It is about whether the notice was served, not what the OP did or did not do - although if it goes to court because of late service, it may well affect whether the OP is sentenced at the fixed penalty level (matters outside of his control).