Looks like I answered my own question after going through the documentation etc further.
I also read that ‘unlocking’ is now supported on some lock devices (maybe not all?) — however this doesn’t seem to work for me, as when editing a device configured in this way, only the name is changeable.
The API suggests that it’s now possible:
https://developer.amazon.com/docs/device-apis/alexa-lockcontroller.html
I wonder if it’s a UK restriction? or perhaps using this method doesn’t use the latest API?