we were told various answers regarding this when we flew to on a 767 short haul, flying with 2 infants.
the final answer we got when we phoned to choose preooked seats was there is one mask per row for infants so we eventually chose seats G&H over several rows.we were not allowed to choose seats online because we had infants on the booking.
when we have flown previous on 757 with 3+3 seat configuration we have always had to sit on one side of the plane, if i remember correct it was the right hand side
if its 90 days tomorrow to your return flight its possible that the seat plan might be open today

ohdear66 wrote:

Hiya! We are flying LGW-LCA in August on 767 v4 and wondered if someone knew whether there are infant oxygen masks on all rows specifically the middle 4 seats on row 1 (c,d,e,f) as I am able to book my seats tomorrow and don't want to worry about being moved because of no infant mask.
Thanks in advance!

It will show you on the seating plan which seats are suitable for lap infants. They're marked with an 'i'. You can select seats online with an infant now, and also check in online.