BA use a four class 777 on this route. There are only three of those based at Gatwick and they all received a refurbishment this year. As mentioned above, there's no guarantee you'll be on one of these aircraft, if one needs maintenance for example, but the chances are good. On the bright side, at least in Premium Economy the seats are only eight across rather than the ten across in Economy.
There are also some eight across rows right at the back of Economy but that's because the plane gets narrower down there.