The "Machynlleth Loop" which is often referred as "The Loop", or the "Mach Loop" is in Wales, UK. It is part of the so called Low Flying Area 7 (LFA-7). More detail about the area can be found on the warplane.co.uk and lowfly.net websites which are an excellent resource for everyone heading towards that area. I added a couple of Google Maps of the area:
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Make sure you have a proper landclass add-on (like UTX Europe) because the area is not presented well in default FSX (coastline completely wrong).
When I make a flight to the Loop (in FSX), I take off from RAF Llandbedr or RAF Valley which are located to the north.