Well I presume it's your parcel? Why are you saying it's the posties fault? You say you have GPS proof you weren't there to recieve it? Who says no one was in at address it was delivered to? Maybe you Mrs had it? Maybe one of your kids had it? It's your word against their word. Who says you aren't lying? What's to say you looked at the tracking, saw the postie had signed for it and maybe you thought "I know i'll pull a fast one and say it was never delivered and the postie signed for it themsleves and decided to keep it"? Then you get free whatever and then you expect the seller to reimburse you. You think this situation never happens? Maybe no one else does this? How about the tables turned on you and you prove you don't have it. You think Joe Public is all honest and innocent?
That's why it's a signed for service, so Royal Mail can protect themselves from situations like this. Royal Mail made the decision to remove that protection (without putting an alternative in place like every other delivery company) so Royal Mail need to bear the responsibility for that.
I'll ask again, what is the point in paying for a signed service with compensation if it never gets signed and there is no compensation?