From a pedantic spotter PoV the "E-7" marketing name is annoying. The next in the official MDS sequence is actually E-12, but no US service has applied for that since none use this derivative.
Boeing should really have known that since the original E-7 was a 707 derivative, later redesignated EC-18B.
The MoD is actually contracting for five 737-7ES, which of course have a different airframe than the 737-8FV ( Poseidon ).
Spotting mode off.