I'm in a similar boat and just starting to look at what we need to do...When we moved from Bosnia to the UK, the dog needed every vaccination known to man, was quarantined in the US for months, and finally was allowed in country. Is it really just a matter of going to the vet, getting this "fit to fly" letter, and bringing her with our luggage?
If I remember, flying her into the US from Bosnia was no prob. We were just waved on through. If there's some vaccination to get her out of the UK, I'm more worried about that and getting her pet passport up to date.