Just talk to your GP. Take a note with you, I have in the past because there is so much to remember and it's easy to not say all if it. You are not conning your GP, anxiety stress depression all have physical effects.
Just say what you want, Doctor I need to be signed off because I'm not coping well. If you've had suicidal thoughts, tell them it will help you to get treatment.
If the GP asks you how long you feel you need say 3 weeks it takes about two for medication to help you.
Now sit quietly for a bit. Push away the thoughts about work and other stuff, play some yoga music, YouTube it. It will help you to relax. Close your eyes and push your tongue to the roof of your mouth.
Every time you start to have anxious thoughts, do that.
Do you have family or friends with you? Company is alwats a good distraction.
Ring journal surgery first thing tomorrow morning 8:30 is usually opening time, tell them your distress and do a site and wait.
Alternatively if you really are having difficulty functioning go to A&E the crisis mental health team will be able to see you.