There is also an option to pick it up from the depot, but they won't let me do this until Wednesday... why the hell not?!
It's usually because the depot they deliver from and the depot you can pick up from are two different places. Usually the 'pick up' depot is centralised and deliveries are carried out from a variety of satellite depots. Either that or they don't actually return it to the depot but keep it on the van. They've taken it back to the delivery depot (or left it on the van) on the assumption that most customers will prefer a redlivery. After a few days they pass it to the pick up depot because, if you haven't asked for a redlivery by then, you're probably going to prefer a pick up.

None of that helps you much and I personally agree that it's not a freindly system but it's actually not unusual. And I wouldn't ride the 50 miles if I were you because it will almost certainly be a waste of your time.

It sucks, but there you have it. If you can speak to the right person you might be able to find out where your package actually is and gho there but I wouldn't count on it.