Hi, 
What exactly are you trying to map here? The land ownership, or everything that is publicly accessible? If it's the former, I don't know that it has a place on the OSM database, unless you wanted to highlight natural areas managed by 'operator=Natural Resources Wales': https://datamap.gov.wales/layers/inspire-nrw:NRW_FOREST_LEGAL_BOUND.

 If it's the latter then an mp-relation with suitable access tags might be more appropriate.
Either way, I can't see any evidence that it's a nature reserve:
https://datamap.gov.wales/layers/inspire-nrw:NRW_LNR or
https://datamap.gov.wales/layers/geonode:nrw_nnr_declarations
and it'd be worth following the local language, i.e: 'name=Parc Coedwig Afan' , 'name:en=Afan Forest Park'
