If you are happy to go to your local Royal Mail delivery office (Must be the delivery / sorting office not the post office) they should be able to give it you as long as you explain that the tracking show delivery was attempted but that they didn’t leave you the Something For You Card.
You would need to take:
1. ID e.g. Driving licence or bank card or passport
2. PLUS also something with your name and address on it such as utility bill / bank statement etc.
3. Your tracking ID .
Sometimes they don’t like to hand it over without the ‘Something for You’ card but if you explain the situation and take the three items above they should give it you in most cases. Worst case they will at least be able to arrange to redeliver it on the following working day.