Read this guide or watch the video on how to create a project using Land Registry data.
Once created you will now be able to add a red line to the interior of the feature.
Select your Land Sale feature and then click on the buffer tool in the top menu.
Select side one or side two depending where you want the boundary, side two will be inside the boundary, select a width and click OK.
A new area will appear under the project. Make sure this is selected.
On the feature details menu that appears select the styling tool.
In fill select the colour and increase the opacity.
You now have a red line on the interior of the boundary.