Where is the best place to post dates for in-store organised play tournaments, particularly UK ones?

There's a specific subsection called 'Imperial Assault Organized Play'

As YogiBaer13 said above, you can post on the OP section of the forum, but I recommend Facebook or other social media. I live in Atlanta, Georgia, USA. We have a separate Facebook group for players of each of FFG's competitive games. Create a group for the game dedicated to your locality, invite everyone who you know that plays and let members do the same. Then, create events for tournaments as they arise and invite all the group members. We also keep pinned posts at the top of each group with info about locations and times of regular game nights. Helps people find other players.