If you're interested in watching low-level military aircraft training in the Mach Loop / Machynlleth Loop, take a look at the following website which gives directions and advice:
MachLoop.co.uk
The Mach Loop in Wales, is a set of valleys, between Dolgellau, and Machynlleth (from which the Mach Loop gets its name), which are regularly used for low level flight training. Mach Loop Wales, Mach Loop Map, Mach Loop Guide, Mach Loop Location, Mach Loop Weather, Mach Loop Frequencies
machloop.co.uk