Yeah you see you paid for it so you /can't/ see it. I didn't so I can. Makes as much sense as anything in copyright nowadays.
Everything the BBC does and has it its archive should be freely available to brits who've paid their tax directly to the BBC in the form of a licensing fee. The fact they withhold anything from the people who paid, in full, whether they wanted to or not for it is morally repugnant.
But my understanding is that unlike the BBC stations, they do not take public funding via the license fee, they sell ad time. If this is true, the thesis that Britons should have free access to it because they paid is not valid.
I'm not in the UK, and I could watch it just fine. Probably because Channel 4 blocked access to it in the UK, where they made use of the laws available to them. These laws likely don't let them enforce worldwide.