The big difference between Watchkeeper and Hermes 450 is the Synthetic aperture radar which makes Watchkeeper useful in typical British weather.
The sad part about the whole Watchkeeper story is the engine. RAE Farnborough put up the initial investment for Norton to develop a Wankel engine for UAV use, to give a practical alternative to the two stroke engines then in use. The vibration of these two stroke engines made adding an alternator very difficult, and this was an essential requirement.
Anyway, once the British Taxpayer had put up the money, and the spin-off company had developed a range of engines suitable for both targets and UAVs, the Israelis bought it, so the proceeds now go abroad.