"Samsung hasn't yet offered a reason as to why the UK (and other markets) will get a lower-powered S4."
Geez. This has been explained to death on various informed Android websites. It has to do with the LTE integration in the Qualcomm processor and the lack thereof in the Exynos.
As far as the "lower-powered" goes, has been explained as well: in big.LITTLE (8 core CPU) configuration you have only 4 cores working at a time, either the faster ones for better performance where needed or the other ones when idling along on trivial tasks. It's not like you'll use all 8 cores at the same time. AFAIK, having 4 cores working at 1.9 GHz sounds like a winner compared to 4 cores at 1.6 GHz. It's not even about the raw speed, it's all in the integration of the cores and their optimization. I bet you anything that no user will be able to discern between the 2 variants the actual difference made by the clocks.
Such poor information coming from a tech site makes one blow a gasket.