Hmm, that's interesting. So BA have put the 777 back on the 198/199 flights have they? They switched to 747 soon after they started this second flight to BOM.
I came on the 199 a month ago and it was still the 747 and it was full. Especially now that it is the Indian travelling season.

But in Y there is a crucial difference between the 777 & the 747: the rows of double seats right at the back. If you get the window side you have got a lot of space between the seat and the fuselage and you can stretch out along the space in front.