"As you were" is a standard military command. It means, literally, "[return] to your previous posture". Soldiers must snap to a formal "attention" position when an officer enters their presence, and the officer issues this command to indicate they may resume their prior activities. The order is also used figuratively to command troops to moderate a temporary excitement. A poster on Urban Dictionary wittily describes the command as "The reset button of life."
In this case the writer means simply that the prevailing hot weather will continue; I think it is a misuse of the phrase, which would be better employed when the mild warmth the British call a heat wave finally breaks.