The last volume (volume 1) of Sell and Murrell's Critical Flora is scheduled for publication next January. There was a copy for sale at an outrageous price - presumably an advanced reading copy.
Wow, I am glad I don't feel the need to have this collectionhttp://www.nhbs.com/series/flora-of-great-britain-and-ireland but please keep your handy as I am posting more and more non-marine plants.
I put off buying it for years....but having taken the plunge, it IS very good, with extremely detailed species accounts. If you don't mind acquiring it a bit at a time, you can often pick up second hand copies and damaged stock online.
I picked up volumes 3-5 when there was a dip in prices for some reason; I got the three volumes for about £150 for the three. Unfortunately that sets the precedent for buying the other volumes.
I just glanced at Amazon UK. There are now Kindle Editions of volumes 3-5 at eyewatering prices. (And people trying to sell physical copies of volume 3 at £600 or £700 - even though it is much cheaper elsewhere.)
Volume 1 is now being advertised for February or March 2018. Cheapest price I see is Summerfield Books at £105, where you can preorder.
I have in on my birthday list