I don' t mind the walk 750XL, but when there are closer options and you are already delayed it does not help matters.
They should have used the quickest and shortest option.
I'm sure you'll find there is a protocol whereby HMRC / Border Force require the Airport to take non-domestic bussed passengers arriving at T3 to a certain gate (or gates).
Unless you know otherwise, you may well have used the quickest and shortest option.