If you have an external doorway that is not one of the ‘standard’ UK door sizes, you may want to try trimming down an external door to meet your exact specification. This is common with internal doors but with external doors you need to be more careful.
Exterior wooden doors can be trimmed, much like their internal door equivalents. Composite and uPVC doors are not so flexible though. Due to the way these doors are constructed, it is not always possible to alter the door size to fit a smaller door frame.
With most external doors, even if you can change the door size yourself it may not be a good idea. It is important that your external doors create a weatherproof, insulated, and secure seal which can be affected by trimming them down. It can also leave the door vulnerable to absorbing moisture. If you need an external door that is not a standard door size it is usually easier to just order a bespoke or made-to-measure door that will fit without adjustment.