+xSIGWX is available from the UK Met Office from 2025 in IWXXM format (via the
SADIS API).

It would be amazing to download & overlay these in SkyDemon!
I'm not sure their inclusion would be worth any more than minimal effort.
Most SD users are likely to flying journeys of a couple of hundred miles at most, at well under FL100 which is the lower limit for these charts.
Whilst seeing the bigger picture can be useful, there are probably more relevant charts for the average SD user - the UK's 215 and 415 for example - if they were available as overlays.