I don’t believe the problem is region-specific. I do have Prometheus metrics from an app in LAX (on host 3944). However, with numerous deploys in LAX I’d say it was ~50% chance of Prometheus metrics working/starting to be polled by Fly.
For LHR I have multiple apps on host 701b that are working, which if I deploy again I’ve no doubt I’ll lose metrics from those 
. Recent LHR deploys I’ve not had much joy with metrics being polled.
I can’t tell you which host aren’t currently working as that information is not available (Feature Request: FLY_HOST environment variable).