I cannot say with absolute certainty what C of E canon, measure or other law permits the use of 1928. It's no great secret the UK Parliament refused to pass it. (As an aside I would like the C of E to be disestablished. The UK Parliament should not be legislating for the C of E). Nonetheless many parishes still have their green 1928 BCPs. Indeed, in my experience if the BCP is used then it's 1928 that's used. I grew up with Choral Evensong every Sunday using 1928. There must be some provision that enables it to be used or a lot of people in the C of E are breaking the law. Perhaps they should build a prison especially for us.