Spotted these in a few Poundland shops now (Leeds, Doncaster) - so I suspect it's national.
Full price is usually £3.99, they go on ebay/Amazon for about £2.99, or in a pack of 8 for about £11/£12.
Busy! Busy! Busy!
Daddy Pig Gets Fit
Out and About
The Best Pet
Piggy in the Middle
Note: There's a varied selection in each shop. Leeds had 7 books apart from "Busy! Busy! Busy!", but Doncaster only had 3 titles but plenty of them.