AIUI one of the issues with "signed for" is that if the recipient organisation receives a large number of signed for letters the they are bagged up for delivery and the organisation signs for the bag. In my case I sent a letter to the probate office signed for, they apparently received it (on the basis that it was signed for) but either because it wasnt actually received or lost somehow at the probate office, they failed to action the item the letter was required for. This issue didnt come to light until weeks later when its non delivery or receipt caused a delay in the granting of probate after much chasing.