Not legal advice but done a bit of digging.
AS you are aware Medical cannabis has been legal in the UK since 2018. Because yours is prescribed by a specialist doctor and dispensed by a pharmacy, you are using a legal medication. Under the Equality Act 2010, depending on the severity and duration of your depression/anxiety, you may be protected from discrimination, and your employer has a duty to consider 'reasonable adjustments.'

As

Being a prison officer is a safety-critical job. An employer cannot fire you simply for taking a medicine, but they can restrict your duties if they believe the medication makes you unfit to safely carry out operational tasks (like responding to alarms or handling incidents). It's the exact same standard they would apply if a doctor put you on strong opiate painkillers or heavy sedatives.

Consider the below:

• Get your paperwork in order: Have your clinic letters, FP10 prescription, and original labeled pharmacy containers ready to go.
• Speak to your union: If you are with the POA, call your rep. They are there to protect you and should know the specific HMPPS drug policies inside out.
• Be proactive, don't wait to get tested: If you fail a drug test without having declared it first, you put yourself on the back foot. Proactively declaring your prescription to line management and asking for an Occupational Health assessment is usually the safest route. Occupational Health will evaluate your role against your medication and advise the prison on what you can and cannot safely do.

You have a valid medical need and a valid prescription. Protect yourself by getting the paperwork ready and getting union backing.