The 380 to me looked very distinctly different- turned south quickly enough and then continued on that heading a la Concorde 09R departure and from where I was it was side on and some way from it so size /height perception was not really relevant unlike when it is overhead.
Certainly was not climbing as quickly as BA 744s on same route or the twins so to me the answer from 10DME that it just isn't that great at initial climb seems about right since as I mentioned earlier it pretty soon caught up with the profile once it was 15-20 miles SW of LHR.
I would also add that I have not seem 380s departing LHR before except (on several occasions) the 1400 EK Dubai flight. I have been in Egham on those occasions so it has gone pretty much right overhead and the slow climb effect didn't seem so pronounced because from underneath the 380 looks huge because the wing is so big and it is therefore extremely hard to make a judgement on how high it is.
( And as for the comment on the A340, well as is well know that is a successor to the Trident for the 'Gripper ' title but a very elegant looking aircraft and also a lovely plane to fly on.
PB