G'day Ido.
As Mike says, there are many different reasons why an instructor would no longer affiliate himself with his original organisation and also reasons why an organisation would no longer affiliate themselves with an instructor.
Below are a couple of URLs for websites that list students of Yip Man and their own subsequent and noted students:
http://www.cebridge.com.au/Ipman/
http://en.wikipedia.org/wiki/Branche...William_Cheung
You will find that their are many William Cheung students no longer affiliating with him, but you will also find that Brian Lewadney is listed on the wikipedia page (follow blue links or scroll down page for William Cheungs students as well as other Wing Chun instructors).
In fact, there are very few people who where instructors or top students while i was assistant instructor at the Melbourne headquarters who are still affiliated with Cheung; most have gone their own way.
That said, i do remember one guy called Brian (don't recall his surname) who turned up to train from Taiwan, or something. He lasted 6 months, was hopeless at forms, dummy, chi sao..... everything, including sparring/fighting, although he did grade to level 3 (of 10). Last i heard he had set up a Wing Chun school and was claiming to be a master, so, yes, there are frauds out there. I don't think this Brian and your Brian are the same, although i had never heard of him until now.