Hello, I am Dr Helen, a UK doctor.
You can self certify for one working week without a doctor's note. Your employer should have the necessary form for you to fill in.
Here is the government advice page about this:
Once you are off for more than 7 days you do need a note from your GP but this shouldn't be a problem as you have already been diagnosed with the same illness that has caused you to be off sick and are waiting for a referral for counselling.
Hope this helps, let me know if you have any more questions.
I joined JustAnswer in June 2013 and am a UK doctor. I have been qualified for 23 years and have widespread experience in GP and cancer care.