For an M.A., I would not contact the professors until I had decided on what university I wanted to attend.
However, the M.A. in the UK usually includes a dissertation/thesis which is written during the last six months of the degree (after two semesters). The dissertation often counts towards 60-70% of your final mark. Most schools would make students choose their topic during the start of the second semester but in my experience it is a great advantage if you start a good few months earlier. It might even be good to start thinking about it now. If you do a thourough read-up on the subject and identify a problem, most professors/future supervisors would be very happy to help you early on. This also means that when the rest of the class chooses their topics, you do not have to fight over attention.
Just my 2 pence..