You'll be contacted, doubtless by letter. I don't think I was given a choice of clinics, just the one nearest to me. Walking distance from home, so that's fine, and only a few weeks ahead. But if your optician says your area offers a choice, I'm sure that's correct. After the initial letter I made all the arrangements online, but I think I could have phoned if I'd preferred to.
The consultant was much more interested in testing me for glaucoma than in the cataract! One eye was done a few weeks later, the other a month after that.
The whole system seems to be a mix of direct NHS provision and contracted private services. People who drone on about 'selling off' the NHS should note that this has been going on for years, and seems to work pretty well.
Everyone says 'you won't feel a thing'. Well, on my first eye, I DID feel something. Not excruciating pain, and perfectly bearable, like an uncomfortable session at the dentist. The doctor said my brain was 'interpreting pressure as pain'. Well, OK, but it felt like pain when it reached me! On my second eye, I was resigned to the same. But when I flinched as the procedure started, the doctor immediately stopped with 'hold on, you shouldn't have felt that at all, I'll give you the stronger eye drops'. THEN I felt nothing.