Naadia, the issue isn't that you are "western" but human, and most men and women would not like the idea of sharing their spouse with someone else.  Did he obtain his citizenship for the UK through you?  Often, and sadly, men simply use women, and hide their true feelings (children, younger wife, or a woman of their own nationality) until they have obtained a western passport.  You mentioned that he is unable to support two wives, are you providing any financial contribution to this relationship?  If so, stop, and let him leave if he wants the girl in his home country.  We ask Allah to provide you with someone better in return.  This is test - be strong and do that which pleases your Lord.  You do not have to bear plural marriage to please Allah, and especially not to please a man.  Use your head and not your heart, and by no means allow yourself to be bullied into polygamy.