I own the Just Flight British Airports East package. I'm very happy with it, except for one thing.
I like to create Ai for my airports but I've has difficulty creating AFCADs for the bases in the package because AFCAD doesn't seem to read them properly and I can't find any proper AFCAD files for download anywhere.
Can anyone help me with these problems?

I don't use any Just Flight products so can't comment specifically regarding your question but in general addon packages come with their own AF2 files and use a unique naming convention; e.g., Flight One's Ultimate Traffic AFCAD files (a thousand or more) are named F1UT_NAME.bgl.

Opening any FS9 airport by searching for the ICAO code in AFCAD2 will bring up a list of all AF2 files in the FS9 directory tree for that airport (the list is priortized by scenery level)

Use a program called ScanAFD to see if there are any duplicate AF2 files in the directory tree.