I have not researched this issue specifically, but do know that there was considerable migration back and forth across the channel at various times, often involving movement of skilled tradesmen from what is now Belgium into southeastern England.Colchester, for example, was full of Flemish weavers.
If I recall correctly, a Fleming was among the company of pilgrims whose tales told on their journey to Canterbury are recounted by Chaucer.
Just as the surname "Fleming" identifies someone recently come from Flanders at the time surnames were being adopted, Wallen / Wallon could very well have been used by an emigre from what is now southern Belgium.
Very few people had surnames at the time of the Norman conquest.To the extent that Wallen marks an emigrant from Wallonia, it would more likely come from the 13th to 15th centuries.
Beyond that, there were also Protestant refugees from this area who came to England in the 16th century and later on.Jean Calvin was originally from Noyon, not far from Wallonia, and his preaching was not without effect there even though the large majority remained Roman Catholic.