What I did: enter PEK as origin, LON,PAR,ROM as destination, "cheapest available for cabin", untick "Only show flights and prices with available seats" to avoid availability distortions (and enter GBP in the currency box to have the fares expressed in GBP).

I am comparing like with like: I am comparing all fares available between PEK and the 3 destinations. I would have thought that looking at the whole of the market rather than arbitrarily restricting the search to one single airline would give you a far better idea as to whether your hypothesis (viz: APD makes flights to London more expensive than to elsewhere in Europe, thereby discouraging tourists) is correct or not.

If tourists can generally fly to London for more or less the same price as to Paris, then clearly your hypothesis is wrong, even if this happened not be true for your favourite airline on your favourite website.