WRT the difference between the approach to R27L versus R27R (based solely on Google/Wikimapia, not actual local knowledge) R27L has obvious buildings
and a highway as well was a perimeter road on the runway centreline, whereas R27R 'only' has car parks (but are these ground-level or multi storey?).
Landing short (even shorter than BA038 achieved) on either would have been disastrous, but R27L would have involved the Air Canada Cargo Facility buildings
and the A30 highway (with occupied vehicles) against 'just' mostly unoccupied vehicles.
http://wikimapia.org/#lat=51.471359&...15&l=0&m=a&v=2
(You can pan and zoom the above view).
An even
earlier touchdown approaching R27L might involve a water landing, whilst R27R appears to be domestic housing . . .
(none of the above seem preferable to reaching the RESA)