So I was up my local GAME store earlier, in Solihull, Birmingham. Right near the door was the plush of the Red Angry Birds bird. They had quite a few there, so it looks like they are just in stock recently.
I picked one up for my uncle and wish I picked one up for myself. Will have to pop back there, or to another GAME somewhere.
No sign of any of the other birds, or the pigs, but I suppose it's watch this space.
These plushes are $11.99 online on the official Rovio site, which works out to about £7.70. So a saving of almost £3, plus whatever postage is.
Sorry about the late edit, but the plush measures about 4½ inches tall.