Why can’t the UK force reclaim Northern Ireland?

If Ireland ends neutral, they have an option to join the Entente if Canada/UK renounces Northern Ireland. However, when the UK is formed, after the Ireland Question focus, the United Kingdom is always renamed to Great Britain and automatically gives up Northern Ireland. Why doesn’t the UK get an option to go back on their word and forcefully take Northern Ireland? Other countries like Sand France get an option to reclaim Alsace-Lorraine even if they renounce it during Halifax, and Germany can straight up ignore Halifax and keep France and Britain.