How can I get the arrow keys back in the centre of the SwiftKey keyboard?
You can reinstall an older version, try one of the following sites to download.

APKMirror
APK4Fun
APKPure

Swiftkey version 7.6.5.7 seems to be the latest version that has the arrow key + thumb keyboard functionality available.
You can download and install the linked .apk file.
After that you need to configure the Google Play store to not auto-update Swiftkey by following these steps:

Open Google Play store
Navigate to the Swiftkey app
Click the three dots in the upper right
Uncheck the box Enable auto-update ⬜.

Disclaimer: I take no responsibility for the apk's on these site.