Hi samjones,
You can self-cert yourself upto 7days before you require a sick-certificate, but if you want to play it safe to satisfy your work, ask for a certificate from your GP, that might possibly be a small charge as you are meant to self-cert upto 7 days, your work can ask you what you was of sick for,but your quite with in your rights to tell them it was an operation which you are not willoing to discuss, and they certainly can't ask your GP or the hospital as you are protected under patient confidentially
Hope all goes well with the op