Brittonic and Gaulish

If the Belgic tribes in Britain settled Britain at a
fairly recent date, taken to usually have been the 2nd or 1st century
B.C., then why is their Brittonic speech not clearly differentiated
from the rest of the British tribes? For clearly we should expect
that the Belgic settlers would speak Gaulish and that that would have
been different from the Brittonic spoken by the native British tribes.

-Michael