The only option available to the customer at checkout is ‘Royal Mail Tracked 48’. If I drag the ‘Royal Mail Tracked 24’ above it in Reflow, then this becomes the default option?
What you are seeing in this screen are the geographical zones your store covers. These are used for offering different shipping options for different locations - e.g. one zone would be UK with cheap shipping, another EU with more expensive shipping.
The way your store is setup, you have several zones covering the same region (United Kingdom). In such scenarios, only the top zone is active, as you have discovered yourself.
What you need to do instead is have a single zone for the UK with multiple Delivery Methods for it. This way people from the UK will always match this delivery zone and will be presented with multiple shipping options to choose from