There are many sources for squawks in the UK online, but a lot of them seem to be based on older lists/information. So, here's the most up-to-date list direct from the UK's Civil Aviation Authority documentation. We'll try and keep this up to date as the documentation is usually reproduced every month, but changes will be rare.

Squawks can be useful for determining where something has come from or is going to in the case of missing route information. Or they can identify a specific type of mission, in the case of things like NATO Air Policing. The emergency squawks are also listed here, of course, but these will alert live on the table on the tracker website, and also hopefully soon into the ADS-B channel on the OARC Discord.

Also remember that each digit is an octal, meaning 0-8, so you'll never see a 9 anywhere.