There's a piece in the Belly Belly articles section on flying whilst pregnant - I was looking up this topic yesterday so I can plan my possible conception month around a UK trip next year! The article says there is no medical reason not to fly as long as you've got a low-risk pregnancy, and most airlines will let you fly until your 35th week.

If my conception efforts are speedily rewarded, I'll probably be doing the long-haul UK trip at 28 and 31 weeks. Hell on wings!