This page is probably only useful to GA pilots in the UK, if you don't know what NATS or AFPEx is then you're in the wrong place!
AFPEx is the UK service offered by NATS as a gateway into the AFTN for filing flight plans etc. It's a Java app that's officially only supported on Windows or Linux accessed via https://www.flightplanningonline.co.uk
Ignoring the odd decision to use Java for this in the first place, frustratingly a recent update means it no longer runs on macOS even with Open Web Start.
There is, however, a relatively simple solution if you're comfortable with the Terminal on macOS.
Download update4j-x.x.x.jar from https://repo.maven.apache.org/maven2/org/update4j/update4j/1.5.9/ and save it in a suitable directory.
From within that directory, run the following command;
java -jar update4j-x.x.x.jar --remote 'https://ts1.flightplanningonline.co.uk/Start/center.cadas'
It will download (this can take a few moments) then run automatically. You can run it again in the future with the same command, and there wont be the delay of the download.
If you want a simple icon to double click on, then put the below in a simple text file script called run_afpex.sh or something and you can open that from Finder;
#!/bin/bash java -jar update4j-x.x.x.jar --remote 'https://ts1.flightplanningonline.co.uk/Start/center.cadas'