OP, DO NOT hand in your notice.
Email your manager and inform them that you are not ready to return to work and that you expect you will be off for several more months. They will need to keep in touch with you but won't force a meeting right away and even then, you can insist on a vid call.
Contact your GP and ask them to provide a fitness to work note signing you off for depression and anxiety for 4 weeks. When that runs out, you ask for another 4 weeks and so on until you are recovered.
Give the Citalopram 8-10 weeks to work and if the side effects of anxiety aren't diminished enough for you to function, go back to GP and ask for a higher dosage and then if still nothing then as for something else.
Expect to be off work for the next 6 months...
The tablets don't always work the first round. It can take 2 or 3 different types until you find the right one that helps you and the right dosage.
If you quit, you won't be entitled to any assistance from the government. If you are on long term sick, you will get some help so that you won't have to worry about money. This is your entitlement remember, you paid in to the system and now you need the support you paid for.
Realistically, plan to be off work for a year unless you find the right fit with medication.
It took me nearly 2 years to be able to function. Insomnia, panic attacks, agoraphobia, upset stomach, loss appetite, night sweats and dark depression were just the tip of the iceberg for me until I got some control back over my broken brain chemistry.
I wasted a year on Sertraline and got worse and worse. I rate Effexor and Mirtazipine highly. Citalopram did nothing for me. You haven't really begun to explore meds and dosages yet so please give yourself time and breathing room to focus on yourself and a recovery.
Don't try to please others or commit to anything. You tell them all that you are unwell, unfit to work and need space to recover. Your job have to check in with you every so often but you can repeat those words and then one day when you too have control back, you can choose to go back or find a new job elsewhere but right now you are not mentally able to make an informed decision. Take care of yourself.