This sounds like a very garbled story to me - it seems very unlikely that men in mail would be told to take it off when facing combat (not quite the same, but just look what happened at Stamford Bridge . . . ).
Gerald of Wales' "The Journey Through Wales" and "The Description of Wales" have been widely misquoted and mistranslated over the years. The Lewis Thorpe translation (Penguin Books 1978) is by far the best modern version and he sets out the description of Welsh fighters (Description, Book 1, chapter 9) in this way:
. . . they have not hesitated to fight without any protection at all against men clad in iron, unarmed against those bearing weapons, on foot against mounted cavalry. . . They use light weapons which do not impede their quick movements, small leather corselets, handfuls of arrows, long spears and round shields.
So the Welsh preferred to fight wearing leather rather than mail in Gerald's time and certainly before that (Description was written in about 1191); the weaponry and tactics tally exactly with what we find in the much earlier Anglo-Saxon Chronicles. If the time scale of your reference was after 1066 the Welsh would be met by Anglo-Norman forces, not English - if before 1066, the English would have been mainly fyrd
, who wore little or no armour anyway.