Because I'm not in the UK. It ain't that easy here.
You forgot about the individual quotes bidder wants. Unless it's the same price everywhere which I doubt.
Over here, we have
www.interparcel.com among others. Very easy to get quotes, you just need the package dimensions and weight, select the courier you want and you get the shipping price. (This is actually much easier to quote than trying to decipher Royal Mail's obtuse pricing). Nothing similar in Oz? If not, that looks like a decent gap in the market.
Not if, as Mike pointed out, you have to be around for the pickup. I can assure you that here in Oz, the PO is much nicer option than a courier for the casual shipper.
Understood. Over here it's a bit different, I prefer to hang around for the courier dude because I'm generally in the office all the time and it's easier for me than driving the 4 miles to the PO. If I'm not there, I leave the package with a neighbour.
You might find you can get some pretty decent pricing with one of the couriers if you're sending lots of packages (uCurrents?) internationally, you also get an API for producing shipping labels. It might work out a bit more expensive than Oz Post, but it's much faster, fully trackable and much more reliable (uRulers?) in my experience. Of course I understand that your mileage may vary.
Back on topic - nice score!