best place to buy a loose diamond

[lazyromantic]

Hi. I live in England and my fiance and I are looking for a diamond. I''d be more than happy with a 0.4ct ( I have tiny fingers so anything that much bigger looks silly) F color VS1 diamond but he''s a perfectionist and wants an E or D IF instead. I was just wondering where would be cheapest to get a loose one. My fiance wants to go to Antwerp to have a look, is this a good idea?
thanks