An interesting letter. Two tidbits:
"Some 64% of employers are reluctant to employ BSL users." -- presumably they felt a bit odd using the term "BSL-speaker". I am dubious, tho; I think the writer may be conflating "someone who can speak/understand BSL" with simply "someone who is deaf, and speaks/understands BSL". The two are no more synonymous than speaking French and being French are.
"Yet we are still struggling, even though there are more BSL users than Gaelic and Welsh speakers combined." -- Interesting, if it's true.