Hi @Mgodby / @chrispybites ,
If your locking condition is dependent on another column, try adding into the editable grid and check if it locks the main field? [Referring to @v-bofeng-msft 's comment].
Now, if the above doesn't work, this is a limitation that Business Rules only work in context of your Form and Editable Grid doesn't activate the Business Rule.
To resolve this, there are 2 options
1. You can create 2 views where Read-Only View contains all the columns and Editable-Grid for only selective columns to change.
2. If you really want to keep 1 view, try Read-Only and then OnSelect of the records, trigger a JavaScript to open a mini-Form/Main-Form as side panel and edit the changes on the fly and close the Form. This way your records in main page View will remain as it is.
Hope this helps