From this Wikipedia page, though, it looks like in UK usage, "inside" would be what I am referring to as the outside. When I say inside, I'm talking about the lane that is closest to the median, but it looks like in the UK the inside might the the lane furthest from the median.

If that's the case, redhorse, passing on the inside (using UK terminology) is sometimes legal and sometimes not, but passing in the lane closest to the median, or at least not in the far right lane, is probably the safest bet.