If you are rebooked, you are not obliged to accept the offered rerouting. There is usually some scope for alternative planning. It is usually best to do the legwork yourself, figure out the options you'd accept, rather than ask the agent to do it.

There are many CX flights every day between HKG and LHR, I cannot say for sure without being there and looking at the loads, but I suspect if you were insistent about it your original route could be restored.

I agree fundamentally, when flying, you have to be flexible.