Implementation of Recommendations
Once a Commission has completed a review, it makes a report to the appropriate Secretary of State who puts legislation to the Parliament implementing the recommendations. Parliament may approve or reject these recommendations, but may not amend them. If Parliament approves the recommendations, then the sovereign makes an Order formalising the boundary changes which are used at the next general election. Any by-elections use the pre-existing boundaries.
Although for many years the legislation gave the Secretary of State power to modify a Commission’s recommendations, this power was never used. This separates boundary making by a combination of structure and convention from those elected from the resulting electoral areas. This significantly reduces any scope for gerrymandering.
Read more about this topic: Boundary Commissions (United Kingdom)