The face of her business is going to change once she has a little person to contend with. If you sit down and run the hard numbers and factor in things like cost of living, insurance (OOP vs NHS), and your salaries relative to that cost of living, how does it factor out? Can she afford to be off while she has the baby, afford the baby and afford you since you cannot work until you get married, apply to adjust status and for employment authorization and receive said authorization? You will be home with the baby, which is great, but is it something that is truly manageable?
What is the contingency plan should something not go as anticipated with the pregnancy? My colleague just unexpectedly delivered her little man, via c-section, 6 weeks early, this past weekend. Luckily, she is in a country that offers proper maternity leave, so she is well looked after and her baby is too. Work colleagues are scrambling because they thought they essentially had a month for her to hand off activities. If your partner ended up in a similar situation or babe ended up in the NICU, is that manageable?
Pregnancy is not typically grounds for an expedite, especially when the petitioner is the American living in the US.
I'm not trying to sound pessimistic, but immigration is not fast, cheap or for the faint of heart. So much planning needs to go into it. I realize she'd be facing similar considerations moving to the UK, but honestly, as a person that had 2 kids in Canada and was able to take a full year of leave, I wouldn't change it for the world. The time I had with my kids to have them hit all the first year milestones and to not have to put them into daycare and expose them to all of the germs was a dream. It also taught me to slow down, enjoy them and realize the days might be long, but the years really are short. The US is very much a rat race when it comes down to work-life balance. She may have a real opportunity to join you and see "the other side" of things.