if the tickets have to be signed for and there is no one in the house when the postman arrives, the post man will leave you a royal mail postcard, with the date, name of person and why it couldnt be delivered. On the other side of the postcard will be the address and opening times of your local Royal Mail collection office. This is where all undelivered post is returned to. You need to take this postcard and proof of identity, ie passport, driving licience to the Royal Mail collection office to collect your undelivered post. It says on the postcard to leave 24 hours between recieving the postcard and going to the collection office.
Dont worry, if this happens , your tickets are quite safe. I reguarly have to go to my local Royal Mail collection office, sometimes it can be up to a week before I have time to go there, and I have never had any problems.