I missed that too, but I heard someone mention it the other day. I'd already swapped my old stamps anyway, just in case I also forgot and left it too late.
I tried to buy 12 stamps in a Post Office yesterday and was told I had to buy them in books of 8.
This seems wrong to me.
I know people will claim that you have to buy biscuits in packets and can't just take one, but that isn't really the same thing.
I once got a PO card through the door telling me that there was a letter addressed to me, to be collected at the local office, which had insufficient postage. I left it for weeks as I had no idea who it was from. On the last day available, I thought I'd better go and collect it, just in case. The excess postage was 8p. Yep, 8p, because it was an A4 envelope. There was only one thing inside - a £50 note.