There is no such thing as a premium bus service, it's always a dreadful experience. I've never been bussed separately
to the aircraft at LHR, and even
from the aircraft back to the terminal has been a mixed experience, it's by no means consistent that as a premium pax I have been bussed separately, neither at T3 nor T5.
I've never been bussed around so much at LHR as in 2012, including to LH flights on a 747!

I therefore take any claims of less bussing with rather a large amount of salt.