Originally Posted by
paperwastage
only if the airline allows you to do so. they may look at TIMATIC for USA-?-? , and say you dont meet the requirements (which is true, because you don't have a covid test yet)
luckily, Finnair allows you skip a covid test if transiting (but that's not exactly what OP wants?)
https://www.finnair.com/us-en/safe-t...afety-measures
Just to be clear, this has nothing to do with the air carrier other than that it is the air carrier which checks your documentation. It is the destination or transit jurisdiction which sets the requirements.
If you transit LHR from the US, you will be required to provide a negative C-19 test result before being permitted to board any carrier. Which takes us back to TIMATIC.