Rolling out IPv6 doesn't mean that IPv4 will be dropped, so existing users and their kit should not be affected. It just means that new customers may be forced to use IPv6 once BT Consumer run out of IPv4 addresses.
In time of course there will be some IPv6-only web servers. That's the users' problem on old kit, not the ISPs. If a customer wants IPv6 capability they will have to buy it. How BT Consumer will handle that is another question, given the ways IPv4 and IPv6 can be set up on user equipment.