Best time of the day for heavies in cluding a few A380s is the early shift :-)
Get down to Heathrow before 6am and you will see quite a few wides coming into land over the next few hours. Sing and Qantas have their A380 first thing, and I think Emerites has theirs in a bit later in the morning (11am). Plus if you get bored with the landings , at about 9am the heavies start to departing, including the A380s (around 10-11am).
Have fun.