Soul records were released over here back in the 60's***, as they were elsewhere all over the world. People heard them, some really liked them, and they then wanted to hear more...simple. Yes there are also some US Military bases over here too, but their influence via Forces radio and imported discs was very limited. Some collectors were also frequenting/communicating with the States buying up vinyl back in the early 70's, and are still active today. There is a hard-core nucleus of collectors still, but Sean is actually probably correct with his, 'there are more Soul collectors in Newton Aycliffe, than in Japan' comment.
I am writing a more comprehensive piece on the Soul collectors and Northern Soul scene over here, but am still working on it three years down the line!
*** - I collect them avidly and will be creating several pages featuring them on AnoraksCorner.com one day.