By Muvija M and William James LONDON (Reuters) -Prime Minister Boris Johnson rejected calls on Monday for Britain to ease visa demands on Ukrainian refugees fleeing conflict, saying Britain was a generous country but it needed to maintain checks on who was arriving. The United Nations estimates that more than 1.5 million people have fled Ukraine since Russia started bombarding its neighbour, with hundreds of thousands pouring into Poland, Romania, Slovakia and elsewhere. Responding to criticism Britain was not doing enough and was far behind its European neighbours in helping address the conti…