From what I can see, The Milligan Wars was a late sequel published late in Keyes life in Japan (only) in 1994. Interestingly, you can get it in Japanese (and Chinese). I suspect is didn't ever get a publication run in the UK or US, and old suggestions that it was available in English in 2015 are probably erroneous. Its only run in the UK seems have been in HB in 2001 (Hamish Hamilton pub.). ISFDB returns no hits for "The Milligan Wars", and searching for Daniel Keyes indicates that it was unpublished (except in a draft format in Japan).
Sorry to be the bearer of bad tidings - but I think an English edition does not exist.