Generally if you get turned away at the destination then the airline is liable for taking you away, so they won't let you fly unless they're sure that you will be accepted at the destination country.
- a valid “immigration document” which satisfactorily establishes identity and nationality or citizenship.
- and, if the person requires a visa, a visa of the required kind or other permission to travel. This visa, or permission, may now be in either physical form, or a digital e- Visa; or
- a valid Electronic Travel Authorisation (ETA), if the person requires one.
https://www.gov.uk/government/publications/document-checks-a...