Your final sentence is key, Trash. Yes, the CAA rightly restrict HAL's charges on the grounds that HAL has substantial market power, such that normal market mechanisms can't prevent it from overcharging. So regulation is required.
The net effect of this is to transfer the economic 'rent' to the airlines, who end up being able to make supra-normal profits. The underlying demand for flights ex-LHR is substantially higher than the available capacity, and BA (and others) are able to charge higher prices than would be possible in a normally functioning market (hence my reference to BA rationing demand by price). This will only intensify as LHR capacity is constrained. That's an enticing prospect for BA in particular, until such a time that the regulators train their attention on the airlines as well as on HAL.