Here in Australia all the major brake shops have books with all pads that have been made. When I sold my brake shop several years ago I left most of those books for the new owner, should not be a problem in the UK. 
They don't tend to be that helpful these days in the parts places in my area. Too lazy to look for you. They just suck their breath through their front teeth and shake their heads.
I tried to get a pair of BMW headlights from a dealership once for a custom I was building. They could not even look up what they had in stock without knowing the chassis number of the BMW I wanted them for!
The internet makes it easy so these days. As long as you know what you need.
I have been running a mopar for 26 years now. I started off with a 318 powered Australian Charger 770 (my first car) in the early 80s and used to try and get parts the way you suggested.
Such as the ignition points in the dizzy. I found they were the same as Aston Martin.
Fram Air Filter was had by comparing.
I also found some rear brake shoes gathering dust in a small motor factors in a little village. But the front pads I had to get from a US parts place 150 miles from me and I finally found some from a Mustang that fitted.
Those Oz cars used to be very hard to get the parts for before the Internet arrived. The Challenger is MUCH easier. Until you start customising it of course.
