A terminal master's degree program from a reputable European program (e.g., Oxford) can be a positive signal as long as you perform well. It is an opportunity to learn research methods, enhance language skills, further your knowledge of history, and define your research interests. Unless your master's degree is from a department that normally grants them "en route" - signaling that you were asked to leave a doctoral program - it should help and not hurt you as an applicant. Admission committees are well aware of other history departments and the types of researchers they produce. It may also be worthwhile to investigate the CVs of faculty and graduate students in the programs you wish to apply to in the future. You will notice that many people at the top U.S. programs obtained master's degrees at other highly regarded programs before earning their PhD. Columbia, Georgetown, LSE, Oxford, Cambridge, and other known programs have an established history of placing their better master's students in their own programs or other similarly ranked programs.
Any reputable U.K. program will place similarly if you are at or near the top of the class (i.e., graduate with Distinction). Also, earning a doctorate at a reputable U.K. program will provide you with ample opportunities to have an academic career in the United States. You will find that many professors at top programs in the U.S. earned their doctorates outside of the country. Usually, you will want to choose the "research" rather than "taught" master's program. The "research" program is for students planning to pursue a PhD or equivalent. English/Scottish programs tend not to post their placements because most of their top graduates stay for the PhD, but you will frequently find that PhDs from the very best programs have master's degrees from these programs.
Georgetown's recent placements:
"Graduate Placements
James Pickett, a May 2008 MAGIC graduate, will begin a PhD program at Princeton University in September 2008. At Princeton, Pickett will concentrate in Eurasian history under the direction of Stephen Kotkin.
Nathaniel Mathews, a prospective 2009 MAGIC graduate, will begin a PhD program at Northwestern University in September 2009.
Evan A. North, a prospective 2009 MAGIC graduate, will begin a PhD program at Georgetown University in September 2009.
Graham Auman Pitts, a prospective 2009 MAGIC graduate, will begin a PhD program at Georgetown University in September 2009. "
NOTE: I recently decided to pursue a PhD in history as opposed to another discipline. This is based on my own research of history programs using techniques I learned when searching for other programs. I do not have specialized knowledge. What I know is strictly from reading admissions websites at the top 50 programs, investigating master's programs in the U.K. and U.S., and pouring over hundreds of CVs. I am interested in a less popular field of history and so finding programs/faculty that fit my needs are a bit more difficult and requires more research. I have no specialized or "inside" knowledge. If I am misguided, feel free to correct me.