Yep it's possible to access public calendars without OAuth, you simply create an API Key in Google Cloud.

1. Create a project in Google Cloud Console
2. Enabled Google Calendar API at https://console.cloud.google.com/apis/api/calendar-json.googleapis.com
3. Generate an API key here: https://console.cloud.google.com/apis/credentials

Then you can access public calendars with simple http calls like this

https://www.googleapis.com/calendar/v3/calendars/[PUBLIC_CALENDAR_ID]/events?key=[API_KEY]

In this case:

https://www.googleapis.com/calendar/v3/calendars/en-gb.christian%23holiday%40group.v.calendar.google.com/events?key=[API_KEY]