First Tip:
Tell us where you want to purchase the ticket / It may surprise and shock you but Which City and Country is an enormous help.
If as a BMI member you mean UK. Then its easy :
http://www.netflights.com. Now owned by Thomas Cook they have 500+ staff in their call centre [Its actually Bigger than BA in Newcastle] including 4 people who do nothing all day but ensure that nobody undercuts them. The founder was a Genius but thats why he sold out for almost $200 Million. Look at The Times Rich List.