I don't think that civil union counts for a marriage; that is what the whole gay marriage issue has been about. You can marry in the US and apply for a spousal visa (no special visa needed to get married, but the foreigner needs to leave like a normal tourist). Or you could pursue a fiance visa for marriage in the US that would let the foreigner stay after the marriage.
I think getting a civil union in England would complicate US immigration, because it is somewhere in the realm of both "too married" for fiance visa and "not married enough" for a spousal visa.
