Mobile ISP's often do this already yes, and its one of the reasons mobile internet is so fricking painful. At least the ones I know of have it opt-out, if I need a public IPv4 I just set another APN on my device, which makes it a bit more bearable for me personally. But that is not really enough for the health of the internet as a whole. When a significant portion of the internets users are crippled, the usefulness of the intertubes suffers in some way or another even for those with proper connectivity.
Anon is right that you could portforward at the ISP in some cases, you just need a new home gateway/router/cpe to support UPNP IGD/NAT-PMP to PCP proxying ;-) And you will still need some 3rd party negotiation of the ports if its going to be useful for other than techies (on the other hand, not as infra/latency/bw critical as a full detour). At least these new devices will also support IPv6!
FWIW, my cable operator have provided me with proper native ipv6 for almost a year at home, as many routed subnets as I need, at work there is ipv6 to the workstation, mom has native at her place, dad has at work (home still a few months out). Cable, FTTH and DSL techs covered. Really UK ISP's need to get their shit together and just deploy IPv6. Its not that hard and others are doing it AT SCALE already.