People (including non-British citizens wishing to visit the UK) who choose not to get vaccinated for whatever reason should not be discriminated against in the form of being prevented from travelling abroad, to or from the UK.
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Current debate focuses around domestic vaccine passports used for everyday life in the UK. However, there seems to be an international effort to place restrictions on international travel in the form of vaccine passports for incoming travellers, as has been seen in the EU (digital 'green' certificates).
As a commitment to liberal principles and the principle of non-coercion, the government should not require a vaccination certificate upon entry to the UK or upon leaving the UK.