You can provide a copy of your flight ticket, boarding pass, and/or the entry/exit stamps in your passport. Sometimes we may ask for a combination of these, depending on the information provided.
Please note that tickets booked for a future date are not acceptable, as we need to see that you have already departed the UK, not just that you are planning to.
If you have departed the UK to a country other than your home country, we may ask for evidence of pre-booked return tickets to your home country.
If you travelled via ferry or train, then we may have to request further information to confirm your departure.
Please raise a query with the Visa Compliance Team if you have any further questions about this.