The 458/5s and 456 projects were separate. 10-carriage trains for the Windsor lines was tendered, and the 458/5 option was selected over lengthening Desiros. AFAIK the transfer of the 456s was just simply SWT taking up the lease once Southern released them. Desiros weren't on the cards. However in the long term, once Waterloo platforms 1-4 are lengthened, additional 10-carriage trains will likely be needed for Surbuban services.