I used a cancellations service when I did mine (passed Sept 2015) - well worth the one off £18.00 payment. I imagine different companies may do things a bit differently, but the one I signed up to locally gives an option to specify any dates you can't do and any particular times of day you prefer (although obviously, the more flexible you are the quicker they find one!).
They also gave me the option to specify whether they can book a test without asking me to confirm (which I was happy with), or otherwise they text you and you need to respond back, within an hour I think it was, if you are happy with the date / time.
It is worth noting that generally, before you can sign up to their service, you must have a test booked - just book the next available date even if it's 2 months away and they will bring it forward.