What are the current COVID-19 testing requirements for returning to the UK from Thailand with a stopover?

The traveler is seeking clarification on return requirements to the UK from Thailand, including testing protocols and the relevance of a stopover in Bahrain. Key insights from comments suggest that fully vaccinated individuals may only need to complete a Passenger Locator Form (PLF) and not necessarily take a PCR test unless specified by the airline. A friend returning via KLM shared that they will not need a test, only proof of vaccination, while entry into the UK requires booking an Lateral Flow Test (LFT) in advance.