I meant "US won't care" as in they won't do anything to stop the merger and they are the only country whose opinion Nissan need worry about. Nissan has been signaled as the brand you go to if even Dodge won't finance you, but they still have a decent sales presence and deep retail channel. Their problem is that they are clearly and obviously still flogging cars that start at ten years old after wasting hundreds of millions of dollars they didn't really have on obvious dead ends like the second generation Titan; and thus can only compete by selling to people with room temperature credit scores and deep discounts. It's basically the situation Chrysler was in in the late 1970s (and again in the early 90s), which was turned around.
My point was more like if Nissan as a whole is threatened from existence (either liquidation or being bought out for the name) and (as the most prominent example for arbitrarily blocking corporate mergers) the CMA tries to block the merger from happening, the solution would be to just ignore them and leave the UK market than jeopardize the entire future of the company. As mentioned above, the Japanese government probably has some hand in suggesting Honda do this if the overall company is in as poor health as they've been saying publicly; and we've already seen said government treat Nissan executives like they were the 9/11 planners when Renault tried a complete takeover.