The airports/airlines have been pushing for testing but the UK government won't entertain the idea so far. The UK Government make the rules not the travel industry. I think their concern is that people who test positive on arrival may still be incubating the virus hence the 14 day quarantine period. I do wonder however if there is scope for a more targeted approach as Germany does e.g. targeting Aragon, Catalonia rather than the whole of Spain, coupled with testing and a shorter quarantine period if the evidence supports it.