Fili, as Davies said, lots of people fly when pregnant. I was just surprised that the Prof was so insistent with me when normally she says that flying isn't considered dangerous. I suppose it is because I lost my second baby on my second trip to the States that month despite being on heparin and aspirin.
Davies, isn't it policy to ground flight staff when they are known to be pregnant? Have you told work yet? It certainly seems to be a very controversial subject with no evidence to back either side and to be fair, I can't remember whether the no-fly order was for just first tri or afterwards as well. I know that ladies who bleed are generally told not to fly in first tri, either.
I think that not only is there a fear that miscarriage can be caused by flying, but if you are at risk you need to consider what you would do if you started miscarrying on a flight? Or on holiday?
Again, there is no proof whatsoever that flying harms pregnancies. The only thing flying is known to do is cause clots and APS also causes clots. Being on heparin and aspirin can counteract this so it may not be a problem at all for you. Why don't you contact St Mary's and ask for advice specifically on flying? You don't need to say when exactly the flight is for...
As for trying before your test...
well, I was at St Mary's and they asked us not to ttc while they came up with a plan. But I'd already had all my tests so ahem... we decided not to prevent and fell pregnant right away. Oops. Sadly, the baby didn't make it (early loss, funny shaped sac). And to be honest, since then nothing was added to my treatment by St Mary's. I was already on heparin and aspirin and it was my local consultant who then put me on progesterone supplements.
So you had best not ask me for advice! I'm a very bad example!
In general, if you are still waiting for tests, which might determine a different treatment plan, then I would say wait. If you will be on the same treatment regardless of what the results are then...
If you wanted to be totally and utterly careful then you could put ttc-ing on hold until both St Mary's are happy and you are back from holiday. But we all know how difficult that would be!
Fili, whatever you decide, I'm sure it will be the right decision for you. Just don't worry once you have made it. You are in good hands now that you have a medical team to support you and we are not here to judge you!
Heart, happy 12 weeks! This really IS a significant milestone!
I am having my hair cut tomorrow morning - for the first time since September! And I have a bob...
My instinct is to cut my hair short again but after giving it some thought I am going to attempt to grow it so that I can pull it back easily once ducky is born. Bye bye lovely bob
and though not as exciting as trips to Thailand, I can imagine that the atmosphere there is quite friendly and relaxed. Do you have to wear a maternity uniform? 
half way there now 
