Compare the number of non stop far east and Chinese routes out of CDG/FRA/AMS and then LHR. London lags behind hugely, now by your impeccable logic, if they can't get LHR, they'll be at LGW. How come so many of them aren't in London at all?
Not necessarily because of slot shortage. For a start LHR has around eleven MEB3 A380s heading east every day, plus other aircraft types, not to mention more A380s from LGW, all offering better schedules and service than the Chinese carriers and BA. Netherlands and Germany on the other hand have capped the capacity of the MEB3 in order to protect the likes of KLM and LH.
Secondly CZ is with Skyteam, and AMS is a major Skyteam hub, so perhaps easier for them to fill the A380 there. Or it could simply be that it's still harder for Chinese to get visas for the UK than for the rest of Europe. Regardless, CZ has a daily 787 to LHR and I'm sure if the demand was there they would substitute an A380 on the route.