Under the Equality Act 2010 if you have a disability you are treated as a protected employee. Telling your current/future employer that you have CF is a good idea as you could lose your protection which otherwise would be available to you under discrimination law.
You can get more information here:
Disability Rights UK - the UK’s leading organisation led by, run by, and working for Disabled people www.disabilityrightsuk.org
Citizens Advice - www.citizensadvice.org.uk