Found yesterday in Poundland in Balham, South West London. Would suggest it may well be available Nationwide...
ZEN Colouring "The Ultimate Collection" 100 designs (by GMC Editors) paperback adult colouring book (RRP £9.99)
Currently for sale online at WH Smith for £6.49, Amazon for £9.48 and Wordery for £6.67.
There were plenty available on the shelf, plus many other similar colouring books.