BA operates three types of 4-class 777 from LHR, which have 13, 14 and 17 seats respectively. The seat maps on your link only show the first two.
You will be on a 17-seater which just has an extra row of 4 seats compared to the 13-seater. 5E and 5F exist because IIRC there are no 3E and 3F (so there are 5 rows of 4 seats A E F K, less 3E 3F and I think 5K, making 17).
Have a look at
this thread which tells you all you need to know about First seating.