How to change the currency displayed in apps if incorrect (Solved)

After moving to the US from the UK, my iPhone apps continued to display the £ currency symbol instead of $. It's frustrated me for months, despite change my iTunes / App Store settings to the US, and adding a US credit card.

After a lot of searching, and trying out suggestions that didn't work (e.g. sign-out of Apple ID), etc... I finally figured it out:

Go to Settings > General > Language & Region > Region

Make sure that the region displays the correct country (this didn't update for me - I think you only do this when you set up a new iPhone). My apps now display the correct currency.