Wow thanks so much for all your responses - you've all been a great help, those I read before deciding, and those I've read after, in figuring out what I need to do. (sorry for delay in replying, spent weekend at in-laws and their computer was not working, and BnB always crashes on my mobile)
Well, I decided to take the clomid and try for the chance of the IUI; ie missing my brother's wedding. In my particular situation, with a diagnosis of very low fertility, I think it was I needed to do, if nothing else than so that I will always know that I've done everything in my power to make this work.
It will absolutely kill me to miss this wedding. But I had just received my AMH reading of 4 (out of 48!?), meaning that my fertility is very low (dimished ovarian reserve). This was highly depressing, and I really now feel that every cycle is immensely precious and that I need to give each cycle the best chance of success that it can possibly get. If we had gone to the wedding and not fallen preg naturally, I would always have a niggling voice telling me that I didn't do everything in my power. Every month I discover more things that can go wrong with a cycle, so I have a very strong sense that time is very limited for me and I can't miss a chance, or miss the chance to improve my odds on a given cycle.
I don't know what my brother will say yet, but he has children and I will convey to him just how much it would mean to us to have a child, and how limited our time is and how low our odds are (of natural preg), and hopefully he will understand in the light of how precious his children are to him.
One other thing I did was to phone our airline and beg to be allowed to change our flights, which they allowed (for a fee) (for which I am so grateful, as they bent their rules) so that the flight to Canada is now on the Friday (wedding on Saturday) and the flight back to London is Sunday. So unless the IUI or a scan needs to fall on that particular Friday (the clinic is closed weekends), there is a chance we could still go to the wedding. Fingers crossed the IUI will be before or after that.
Thanks again to everyone!! and good luck and baby dust to everyone!! xxx
PS Edit to answer Velo's questions - The clinic want to move me from the IUI stream to the IVF stream asap because of the urgency of my situation, and IUI success rates are so much lower than those for IVF. This is fine with me, because I'd rather do IVF asap given the success rates.