Originally Posted by chucko

I think it was simply because T4 is overbooked these days. If my inbound flight from Seattle ever arrives early at T4, it's not really a time-saver---we either sit on the tarmac for a half-hour or so until a gate opens up, or park away from T4 and ride the bus in to the terminal. Since BA already had a presence at T1, it is the logical place to run extra long-haul flights from, at least until T5 is finished.