I may have the wrong end of the stick here, but I think the OP is asking about going from NHS to US health care. And she wants to know if a US employee insurance plan will accept that she's had health insurance which covered her pre-existing condition (NHS).
As I say, I may have it bass ackwards because her profile says she's already in the US "for now". But I just want to point out, that it's unlikely that any foreign health programme (even UK private) will be considered for US continuity eligibility. Case in point, your US employer's insurance won't even cover you in another state, unless it's an emergency or life-threatening illness; don't even ask them to reimburse for foreign treatment. How's that for restrictive? Hope Obama, et. al. gets US health care figured out someday.