There is no narrowing of the seats on this row (or any row), and row 12 does have noticeably more knee room, as opposed to leg room. Row 12 does recline, row 10 does not. Depending on the route (i.e. high status or not) then the middle seat may be more likely to be occupied in row 12, but both rows could be subject to curtain roulette. In that situation I would choose 12A but I would keep an eye on it just in case the curtain moves or there is an aircraft change, LGW only has a few of these aircraft.