Can anyone please clarifyconfirm this? My understanding from speaking with former Sea Vixen pilots is that due to the transonic characteristics of the Vixen, they were never able to exceed Mach 1.0 in level flight. However, Mach 1.2 was normally achievable in a dive of approx 30 degrees.

That would be true of a normal Vixen. Foxy is far from normal. To be wholey accurate she is a demilitarised D.3. She doesn’t have the near 1 ton of radar in her nose nor the very heavy and cumbersome radio fit of her service career. During her conversion to D.3 status FRa also added afew modifications to her systems, effectively ‘tweaking’ airframe and engine performance.
All this adds up to her being supersonic (Mach 1.23 as I remember) in level flight at her optimum cruise altitude and lighter on her feet.