People who use BSL as their first or preferred language are between 50 to 145k depending on whose stats you're looking at (there have been issues with the census and Deaf communities due to lack of government support, the last census was awful for this). To compare - Gaelic users are around 60k by similar counts which does have a GCSE. Both are official languages recognized by the UK [BSL has had official recognition for 10 years, many argue it hasn't done much] and are meant to get the same status but they don't - getting information in BSL is quite difficult and no qualifications within the UK recognized system, just separate charities who have working very hard on getting a working system and a GCSE for BSL for many years. Having one will not only raise a lot of awareness and recognition, as well as get more people learning BSL, but it will get Deaf young people, who are more likely to be NEETs and have trouble accessing further education & employment, more recognition as well. While locally working on this, a national one would be nice.
Cochlear implants hasn't really affected the numbers as it is only useful for certain types of deafness and many with them also use and prefer BSL (and speech still doesn't come across well with those that do work, usually only to the extent of making lipreading easier).