Better check your country's Board of Nursing first of what are the requirements if you are planning to work here in US. I am assuming that you want to be a CRNA? If you are planning to get your education here in US, as far as I know, most of the universities requires you to have a minimal US ICU experience. The nurses ICU responsibilities/experiences differs from each country, and even most US hospitals doesn't recognize your experience so least of all the US universities. No matter what you're ICU experience back home when you apply here to work, most US hospital will still give you a good amount of orientation, both knowledge and skills, hospital's policies etc...But then again you can write an email to the directors of the universities you are interested in, almost all of them do answer back, if you have anymore questions because they know better than anyone else.
Each country differs in grading system, and US schools
uses GPA calculations from a 4.0. 4.0 is the highest grade that you can get equivalent to an A, and 3.0 is a B. And your transcripts after you get your Bachelors will be evaluated by the US universities to convert your grade to US...it's like metric system to english or vice versa, or spanish language to english language..it's all about conversion, so US universities will have an idea of what your grade was in UK, or if your Bachelors back home is an equivalent to US BSN..you can ask your university too of what to do to clarify things.