For re-entry into the UK, there are no current COVID-19 travel restrictions in force. This includes eliminating the requirement to fill out a passenger locator form and testing non-vaccinated travellers upon their arrival in the UK.
You might still be subject to restrictions in the country you're visiting, and wearing a face mask might be needed on flights, cruise ships, and at resorts.
Covid vaccine status
You can demonstrate your vaccination status via the NHS app
Click here for foreign travel advice including country entry requirements and coronavirus information. Check the entry criteria for the country you're visiting and, if necessary, get tested and complete a pre-departure form. Please note that even if you have been vaccinated, this may still be required depending on the county that you are travelling to.
The FCDO link above will tell you whether you need to fill out a pre-departure form and/or get tested before you travel or after you get to your destination — the dates and types of tests that are accepted vary in each country.
You are responsible for making sure you satisfy all entry requirements, including arranging and paying for tests as necessary. Runway Travel can assist you to help you find companies that provide this service.