Guardian article which says:
General rate of birth defects in Pakistanis: 3%
Rate where couple are first cousins: 6%
General rate of birth defects in Brits: 2%
Rate when the woman is over 34: 4%
(The article headline only covers the cousin marriage aspect.)
My reading of the article does suggest a naturally higher chance birth defects - 2% Vs 3%.
"For too long, we have been a passively tolerant society, saying to our citizens 'as long as you obey the law, we will leave you alone'" - David Cameron, UK Prime Minister. 13 May 2015.