Mrsk, I don't think there is anything in the booking bloods that shows whether your pregnancy will be successful or not. They don't check your beta hcg until your 12w nt scan.
In my local epu it depends on the sonographer whether they show you the screen or not. Usually when you come in you always get scanned. It is part of their procedure: see nurse, have scan, see doctor.
So please don't be worried if you ever go in and the sonographer chooses not to show you the screen right away. That does not mean that they think it is all over!
Embo, I asked whether I could book in with the midwives later since I've been through it twice before and haven't bothered since. I think I went after ten weeks. I just called them up and they gave me an appointment a few days later.
As Davies mentioned, they check your blood type and whether you are HIV positive, have syphilis or hepatitis (cant remember which type) and whether you are immune to rubella. If you've had that before most likely nothing will have changed. The only variable is the blood count but again if you are feeling well a two week delay probably won't show up any surprises (like newly developed anaemia).
If you've never booked in it is a good idea to go because they give you all the bunf about being pregnant, tell you about all the resources available to you and answer any questions you may have. Generally, midwives are really nice people and I always leave with a warm fuzzy feeling inside.
Heart, I select all/copy all my longer posts these days because I get too frustrated when I get kicked out or something