Any question answered...

Are there domestic heat pump dehumidifiers?

Yes, most of the decent ones. The fundamental principle of a refrigerant cycle dehumidifier is that it's a heat pump, moving latent heat of vaporisation from the condensate to the ambient air. The Trotec TTK series are good. As heat pumps, they have an efficiency of about 2 (e.g. 250W electricity in, 500W thermal out), although that obviously varies depending on the initial ambient humidity and the target humidity.