I asked relatives of ours, they travel to UK about 3 to 4 times a year thru. S'pore with Singapore Airline on the exact time as yours. They said the transit time is tight but they have never missed the conecting 11.35 flight to London. The A380 flight always leaves about 15 mins late.
In fact our family have booked the same flight time in August, I'm not too concerned after speaking to our relatives. We'll make sure we don't stray, and go right ahead to the boarding gate.
Something to note is you'll get your boarding pass to London when you check in in HK. But you still need to go thru Security again before boarding the 11.35 flight.
Hope this helps.