Not explicit policy at present, but any new diesel-only units would have to be retired before 2050 because of the legally binding carbon budget for the year 2050. All diesel units will have to be retired by then, so ordering new diesel units now guarantees a short life, which increases cost whether you buy outright or lease them.
It's also important to remember that this tender specifies all units either need to be decarbonised from the start, or designed to make decarbonisation later in life easy, so you can see this realisation working through in this tender.