I'm not 100% sure, but I believe that Mr Muscle glass cleaner is the same as Windex. At least the ingredient list sure looks similar.
SC Johnson sells Windex over here in NA at almost all grocery, hardware, auto and practically any other kind of general merchandise stores. I have about 3 bottles here at home. At work we go through cases of it. It's a great general purpose cleaner. I often see people wiping phones and computer keyboards with it that they share with others. Some bottles say they come with Ammonia D and others don't, there's about 10 different varieties. It's mostly water and Isopropyl Alcohol with a few other minor things to fancy it up some.
SC Johnson sell Mr Muscle in the UK and since I've never seen Mr Muscle glass cleaner here, I would assume it's your equivalent.
Ammonia, on the other hand, can be purchased anywhere we buy laundry detergent, in liter or gallon jugs. I'm surprised it's hard to get over there.