Then we need to define what a Deltwing plane is, my friend.
If you only accept jet driven...okey.
But even then, the Avro plane is much influenced by the theoretical work of Lippisch and Steyer. Both wrote dozens of articles to the RLM in order to prove their conceptional way. Lippisch was way more successive, we know. After VE-day, the US shared some of the papers with the UK. At the time, the Avro-707 was in concept phase, the Deltawing was not only known by researchers because of the work of Lippisch but he also made subsequent wind tunnel tests of models, prooving, that his design may reduce the drag in trans and subsonic speeds compared to normal or even swept wing layout.
Keep in mind that his DM-1 glider directly led to the XF-92 Deltawing plane (first flight was on september 18th, 1948), which in your definition is the worlds first Delta.