So yes, that's gone down to a A319 and is currently full. I think there is a good chance it will be changed to an A321, However the seating issue should resolve itself over time, and at this point I would not assume that you are bumped, unless you get a message to that effect from BA. The one thing you could do now is split the reservation into 2 and 3 passengers, and then 2 of the passengers would be able to get seats now, but that comes with risks too, such as ending up on two different flights back. There are other services from ZRH to LHR and LCY with seats earlier in the day.

It is a long way off, and a lot could change. BA remain responsible for getting you back from ZRH to London.