There's no need for a Mk for the Vauxhall Omega, there was only one generation. Same with the Nova. The Belmont (sedan version of the Astra Mk2) was also only one generation and as such doesn't have a Mk field either.

In addition, it doesn't make any sense to call the Vauxhall Omega the Mk2 as the preceding Omega [A] is the Vauxhall Carlton Mk3. The Vauxhall Carlton Mk1 and Mk2 are known as the Opel Rekord E1 and E2.