You can buy whenever you like, but you have to factor in the early termination fees (50% of remaining lease payments). Whilst VWFS insisted it had to be kept separate they did reduce their final offer to fully offset the termination fee.
They came down significantly over a few weeks of discussions. No doubt helped by the great lease deal too, but the total I paid for my 245 (with dsg, panoramic roof, keyless and a couple of smaller options) was almost exactly £20k. That includes the total paid under the lease plus what I paid to the buy it outright.
I think there was a big element of luck in timing as used car values were in freefall then and they clearly need to get some cash in.