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The UK does not issue visas on arrival. You will need a visa to leave the airport. From https://www.gov.uk/government/publications/uk-visa-requirements-list-for-carriers/uk-visa-requirements-for-international-carriers
βHolders of non-national and refugee travel documents
If the passenger holds a refugee travel document issued by the UK they do not need a visa. If the passenger holds any other non-national or refugee travel document they need a visa to enter the UK.
Whether holders of non-national and refugee travel documents require a direct airside transit visa (DATV) depends on their original nationality, and whether they qualify for one of the exemptions listedβ