I am on an older 2Degrees account (from their Snap days), and I pay them for a static IPv4.  On such accounts, the static address payment also gets you a static IPv6 /56 assignment.  In my opinion, IPv6 is virtually useless unless it is static.  There are far too many places in a network where you need to configure an IPv6 prefix that do not support getting that prefix delegated from the router.  All those bits of configuration will be broken whenever the delegated prefix got changed.  The reason for not having support for IPv6 prefix delegation in everything is that by design, IPv6 addresses are supposed to be static.  I have never understood why a lot of ISPs are using dynamic IPv6 prefixes.  And as well, even if all software using IPv6 prefixes did support delegation, when the delegated prefix changed, the network would be disrupted for some time as the delegated prefix was propagated.  All existing IPv6 connections throughout the network would be broken.  When anyone using IPv6 needs to change their prefix, such as when changing ISPs, it has to be done in a carefully planned way at a specific planned time so that the entire IPv6 network can be shut down and restarted on the new prefix.  Random changes of prefix are impossible to work with.

There is absolutely no need for ISPs to use dynamic IPv6 prefix assignments - the whole point of the design of IPv6 is to allow enough addresses for anyone to have as many as they could ever need, so they can all be static.  But ISPs seem to have decided that it is too much trouble for them to get this right and change their customer address assignment systems to allow dynamic IPv4 addresses at the same time as static IPv6 addresses.  I can see that happening for a few months until they updated the software, but no longer than that.  And preferably, they should not have been offering IPv6 until they could do it right.