Not really my part of the world. So no local knowledge. But since I started off the VHF discussion, probably appropriate to respond.
Looks like responsibility is Gloucester Harbour Trust to the tidal limit then the EA's.
Environment Agency - Access
(maps are too low resolution to be useful!)
It looks like boats may be able to get as far as Hay-on-Wye but dont quote me on that, but down to Chepstow looks like few other boats would be around. I'd expect VHF is probably fairly worthless down to at least Chepstow. And according to this: http://www.gloucesterharbourtrustees...ncejun2011.pdf
you should assume useless in the whole of the Wye for anyone other than very local boats to hear you. The valley looks quite steep sided which limits the value of VHF which is pretty much line of sight. Certainly wouldn't want to rely on it as my only means of seeking assistance if something was to go wrong.
I'm sure someone local will be along to confirm but it doesn't look like you need a licence to use the Wye. BUT you may need to pay to launch...