Monday: BG almost certainly won't do it eithwit a full service contract, which you absolutely do not want.
Those valves last forever, so do not worry about changing it. It is not drsidesi as a user control, but for service engineers.
My advice?
You seem smart enough to to be concerned about safety, gas the firebern serviced on regular basis, it within the last year, or thereabouts.
If not, then find an experienced and gas fire registered Gas Safe guy, and ask him for a service. Explain about the service valve, and he will check it and replace if necessary.
Although it would be wrong for me to say how others should operate, on my charging structure it would cost almost as much for a new valve as it would a fire service and check. And changing the valve at the same time would not be a great deal. If the missing decor ring is not bothering you, then don't bother changing it, unless necessary.
DO NOT GET BG TO DO IT
For a free check, contact Cadent and explain your concern.