`  ==New Outline of British History==   Recently, Dr. Goldstein said.  ``One tends to think of England as Anglo-Saxon,`` ``But we show quite clearly there was not complete replacement of existing populations by either Anglo-Saxons or Danes. It looks like the Celts did hold out.``  British historians have generally emphasized the Roman and Anglo-Saxon contributions to English culture at the expense of the Celtic. A recent history of Britain, ``The Isles`` by Norman Davies, tried to redress the balance. The Celts were ignored, he noted, in part because no documentary histories remain, the Celts having regarded writing as a threat to their oral traditions. Generations of historians saw British history as beginning with Roman invasions of the first century A.D. and indeed identified with the Romans rather than the defeated Celts.  ``So long as classical education and classical prejudices prevailed, educated Englishmen inevitably saw ancient Britain as an alien land,`` Dr. Davies writes. The new survey indicates that the genetic contribution of the Celts has been as much underestimated as their historical legacy.  Dr. Davies said in an interview that ``traditionally, historians thought in terms of invasions: the Celts took over the islands, then the Romans, then the Anglo-Saxons.``  ``It now seems much more likely that the resident population doesn't change as much as thought,`` he continued. ``The people stay put but are reculturalized by some new dominant culture.```