Even if you don't go single stack. Being native IPv6 right through to the end of the ISP network (WAN) for now will benefit us all.
As for one thing addresses which aren't translating (bridging) very well between IPv4 and IPv6 are adding additional load, due to not being able to simplify processing by routers and also introducing opportunities for black hole connections. For the average user it results in more load on routers at the ISP, which then impacts our own internet connection performance and also access to services which aren't bridged well due to non-native end-to-end ipv6 especially when using the features which are only in IPv6.
The area where the problem is most noticeable is when using application protocols which are embedding Internet-layer addresses, where new address format are able to conflict its existing protocol syntax. An example of where this occurs is using FTP and NTP, there's likely others where this is true. Also allows for the possibilities of issues with PMTUD (Path MTU Discovery), where it completes the TCP 3-way handshake successfully but then proceeds to hang when data is transferred over it as a black hole connection.
Additionally BT's Digital Voice (DV) is an example of IP-Telephony (VOIP) which is affected by address translation, so having full native dual stack is a benefit here. This thus means the address translation for IPv6 is within their own network no longer an issue. Especially important given the upcoming switch off (close down) of PTSN during December 2025 which is the year after next!
BT can correct this by enabling their current DNS server's (if they are compatible) to connect to the network with both IPv4 and IPv6. As well as configuring the service which the Smart Hubs and Home Hubs connect to, so that it is specifying both IPv4 and IPv6 IP addresses of the DNS servers.
To get a better idea checkout this article - https://en.wikipedia.org/wiki/IPv6.
To see the current situation for BT as ISP - https://test-ipv6.com/.
