Usually MA courses are on site but there are several universities which offer a combination of online and onsite degrees, and some which offer degrees by distance learning. The Open University for example is a long established UK university dedicated to distance learning. They have many undergraduate and postgraduate courses including an MA in Education. Their website is at http://www.open.ac.uk/
And browsing the Net just now I came across a UK College that offers MA TESOL which can be completed entirely online. I really don’t know anything about them or their programs but here’s the link for you to take a look: http://www.newschool.edu/matesol/
I also read that Alliant International University in California http://www.alliant.edu
offers their MA in TESOL in an online format. Again I don’t know anything about them but you could look into it.
In fact you’ll find that if you do a search by key words there are many MA TESOL programs advertised. Just be careful which one you choose. Good luck!