Hi sturmae and welcome to TFF!
I have read your whole thread and believe you currently still do not have fish or ammonia, either one. Its sounds like last night you were about to go get mature media from a friend 3 min away. Did you do that?
If not, I would suggest you hold off on that offer for just a bit but be sure to keep the offer alive! If you don't have the right kind of ammonia yet then you won't have food for the bacteria you are getting from the friend and they will start dying off the next day or two. You still won't want to get fish yet as the donation of mature media from his/her filter won't be enough yet to get your filter fully cycled, its more of a thing to speed up the fishless cycle.
So back to the ammonia problem. This is a very, very common problem in our forum. You have to be very, very persistent as you go about finding the right ammonia but its well worth the effort! Each place you go, you have to try at least a couple things: first, its good to take your time and go directly to all the store sections where it might be and look carefully. This is because we've sometimes heard the strange case that store workers have been told to -not- tell that its there (something about it being used for bad things or something.) But then, exactly the opposite, in some cases as said above the store will withhold it and you have to ask for them to bring it out!
Even though it doesn't seem that way so far, you are lucky in that to me it seems most of the UK members eventually do find the correct ammonia and often other UKers can help them out with the actual shops. Its sometimes worse in the usa and definately worst of all in australia. You can do searches here on TFF for "ammonia" and buried among the results might be other shops mentioned. Boots, Homebase, Wilkinsons, small hardware stores, independent chemists, something called "Jeyes Kleen Off ammonia"... these are all success outcomes in UK that I've read in posts here.
Anyway, keep at it sturmae! I know all this seems strange and complicated, especially compared to the first comments from the fish store, but what's happened is you've stumbled onto a forum of true hobbyists and in the long run that very lucky!
~~waterdrop~~