I just figured this out, and wanted to share my step by step on how to get IPv6 to work on the Sophos UTM router:
1- Connect 2 ethernet cables from the Comcast Modem (I'm using a DPC3939B Wireless Gateway), 1 for the static IP routing, and 1 for IPv6 Connection
2 - On your Sophos UTM router (or any router), configure any interface you just connected for your static IP block, the other interface will need to be set as dhcp for both IPV4 and IPV6.
3 - Access the webpage for the Comcast Modem (http://10.1.10.1), then go to Gateway -> Connection -> Local IP Configuration.
4 - Uncheck the box that says "Stateful (Use DHCP server)" (See below)
5 - Go back to your Sophos UTM, and go to the IPV6 tab under Interfaces
6 - Turn on the switch for IPV6 (see below)
7 - After a minute or so, refresh the tab, and you should see your Native IPV6 block!
8 - Now go to the Prefix Advertisement tab, and add a prefix. Choose the interface you set for IPV6. It should autopopulate the DNS server already. Then Click OK. (See below):
9 - Go to a workstation on the same network, and do a release and renew, and you should get an IPv6 address in that subnet. Try test-ipv6.com and see if you can pass the tests now!