How do I unlock the community research branch?

Playing as west Seaxe I've conquered most of the South of England. I have several cathedrals and level 5 abbeys. However the community research branch is still not unlocked.

What do I need to do to unlock it?

After playing the game I had the same issue. When looking on the steam forum I found out what settlements can help with this.

• Ioua, Druim Alban (island on the west coast of Scotland),
• Dun Patraic, Dal Fiatach (pretty much easternmost tip of Ireland),
• Inis Faithlenn, Iarmuma (southwest Ireland),
• Achadh Bo, Osraige (southeast central Ireland).

There is also an explenation linked as to why this is such a "hard to get" tech tree

The answer is that England and Wales were not centers of monastic learning to the same degree that Ireland or Scotland (through Gaelic colonists) were. I believe the Irish have no access to the market techs, as there are no market hall type settlements in Ireland, without attacking Great Britain. The Irish were learrned but they did not have access to the same flows of commerce that England in particular did. Their trade was often in cultural goods captured by other techs such as the monastic techs.