@GemmaJen I didn't explicitly tell me boss but 'hinted' enough that he knew from about 7 weeks because I was so sick. When I got signed off at 9 weeks, my Dr put on my sick note that I was pregnant so he had something in writing (as did HR) then, my GP said because it's down as pregnancy sickness they can't hold it against you.
I told him with the actual words the week after our 12 week scan when I went back to work, and I notified him in writing this week, after we got our screening results back (as you're meant to have notified them in writing for the legalities of having the time off and things). I have stressed when I spoke to them and again when I put it in writing that it is to be kept strictly confidential, as we've not told anyone other than my parents and now my Nan, and one friend.
Our team is ridiculously busy as well but he's been really nice about it and said some things are more important than work and not to worry about it, they've got things covered whilst I'm not fully up to my normal levels! I wouldn't worry about your boss - yours and babies health comes first, just tell them and make sure it's confidential.
I had my ultrasound date come through 2 weeks after my GP requested it, it just came in the post. I had my booking appointment when I was 10.5 weeks, which was at my GP surgery with the local midwives (I'm South Oxfordshire, all NHS trusts seem to do it differently!). My ultrasound was at the hospital, and after the scan the midwife there gave me a form to go and book my 20 week scan in there and then at the reception desk, which was a good way of doing it (we've got the 1st appointment of the day).