There is a reason my handle is BA0197. I find the later flight much more enjoyable than the earlier one for a handful or reasons:

I always prefer to have lunch in the lounge compared to breakfast.
The immigration lines at IAH are not as busy.
The BA195 tends to be delayed more often then the later flight.
I also like arriving at 6:30p.m. as I find I can cope better the next day with jet lag. In fact I usually have none if I take the BA0197.

The only drag about the flight is that it is almost always packed, including F.

And yes the 12F 772 aircraft with RR engines do make regular appearances on the 197. I was once told that the aircraft for the 197 comes from HKG quite often and a LHR-PEK flight would require a RR fitted 777. Do refer to this link: http://www.thebasource.com/flighttracker.html and type BA197 in the box. You will find that the flight has been nearly 100% successful of obtaining a NF aircraft (apart from the 5th of July).