In the modern British workplace, you can’t be fired or discriminated against in any way on the grounds that you’re HIV positive. That would be a criminal offence. Notifying your employer isn’t a legal requirement, but if you do chose to disclose your status and are met with anything other than support, confidentiality and reasonable adjustments, then your employer has almost certainly broken the law.
Part of the Equalities Act 2010 was created to lessen its impact in employment and ensure disclosure wouldn’t jeopardise careers or future opportunities. Under the legislature, people with HIV cannot be treated less favourably under any circumstances. But thanks to the stigma and misunderstanding surrounding HIV, some have found it wholly ineffectual.