A mountain climbing thread! Yay!
Alison Hargreaves climbed the north face of Eiger when 6 months pregnant back in 1988. She was being cautious - she originally planned for Denali, but heard that high altitude is bad for the pregnancy, so she chose a smaller mountain.
My point is that there are no specific rules for climbing when pregnant.
It does sound like you're prepared. Hiking boots are necessary, as are snacks, food and water. Obviously. I especially like your idea of bringing a bivi bag (your bag that turns into a bright orange sort of tent). If you do get more exhausted than you expected, you'll at least not get worse due to exposure if you need to make a prolonged rest stop.
I'm not British and never climbed Snowdon - pregnant or otherwise. I did do some rock climbing - both indoors and outdoors throughout my pregnancy. I even climbed the world's highest artificial wall in Copenhagen at 30 weeks pregnant. I was careful, I only did top-roping or climbed second (and had experienced climbers lead within their comfort zone and not at their max level), took plenty of tests and so on. My main problem was the fatigue. I lost physical stamina really fast. Carrying heavy backpacks was also a problem for me, so when planning and preparing, you can also consider how much weight goes in your pack. Or maybe your travelling partner can carry a little of your load - I sometimes asked this of my climbing partners. But if you don't feel nauseous and fatigued, I wouldn't think hiking up Snowdon along some of the popular, easier paths should be a problem. I can see from your comments that you originally planned on doing it alone, but decided on finding someone to do it with, after all. I definitely recommend not going alone. Especially when pregnant, but also when you're not pregnant. I'd focus on finding someone with a good fitness level, and ideally someone with a little more mountain experience than you seem to have (sorry, I know you've been annoyed with everyone who pointed out your lack of mountain climbing experience so far).
Enjoy your trip! - I'm only a little jealous, Snowdon sounds like a lovely mountain.