The IMC rated pilot requires 1800m for take-off and landing. They may fly VFR in 1500m visibility. They may fly IFR in 0 visibility (cloud).
There is no contradiction.
Depart in a visibility of 1800m or more, fly enroute in a visibility of 1500m or more and land in a visibility of 1800m or more will satisfy VMC for the IMC rated pilot.
Of course, his Friend with a JAA PPL from a non-UK country can depart, fly enroute and land all with 1500m visibility.....obvously a much better pilot.
While talking Special VFR don't forget that in many control zones within the UK, a UK PPL without IMC can fly special VFR in an entry/exit land notified for the purposes of Schedule 8 with 3Km visibilty.
They don't half make it simple.
Oh and another - the UK has a difference with ICAO regarding VMC in class D - the clear of cloud bit. Don't forget when abroad that a special VFR clearance may be required in 50K visibility if one can't remain 1000ft vertically from cloud. Since ICAO requirements say no less than 500ft enroute - this means that a ceiling of less than 1500ft requires a special VFR clearance to enter or exit a zone!!
Regards,
DFC