Can you, if you have not already, set debug on for dhcp6c. Restart Opnsense and and see what happens. When ipv6 fails send send me the \var\log\dhcp.log to take a look at.
The WAN v6 address should only be a link local fe* address, as Sky do not give out a global v6 address, so that means very little. The key is looking at the dhcp6c logs.I've emailed skyeci ( user on the Skyuser forum ) who does much of the testing for Sky ipv6 for me and he reports no issues with 18.7.10.
Jan 25 13:31:56 dhcpd: Sending Reply to fe80::1028:4bee:96e2:b7c port 546Jan 25 13:31:56 dhcpd: Wrote 13 NA, 0 TA, 0 PD leases to lease file.
You could cut and then paste using the insert code button above like thisCode: [Select]Jan 25 13:31:56 dhcpd: Sending Reply to fe80::1028:4bee:96e2:b7c port 546Jan 25 13:31:56 dhcpd: Wrote 13 NA, 0 TA, 0 PD leases to lease file.