I'm facing a similar situation, in particular with Avast One VPN to access all media streaming service from the UK: iPlayer, itv Hub, channel 4, etc.
I'm a UK citizen and fee paying customer of BBC License and I'm in the US right now for vacation, and I have used many freely available VPNs in the past to access UK streaming services when I'm abroad (just recently as last December).
I've decided to buy Avast One to keep my laptops safe (as well as their advertised VPN feature) just recently.
But, as the original poster has said I'm also unable to watch any of the UK services with Avast One VPN. HOWEVER, I am able to watch them if I use any of the competior's VPN, e.g. ExpressVPN works fine.
With Avast One VPN turned on, when I go to
https://whatismyipaddress.com it tells me I'm in London and yet none of the UK steaming services worked! So, there is something definitely wrong with Avast's implementation of VPN!