When I did an art therapy taster day thing at Derby uni they said to get on the course you would need 2 years of relevant work experience and a proper art portfolio to demonstrate your skills at art. So no, you couldn't do it right now. The masters is also unfunded and lasts 2 years during which time you will have to pay for private therapy as well so very expensive. Lastly, jobs in this area aren't the easiest to find either, as you can imagine when budgets are cut jobs like art therapists are probably seen as less important than, for example, a psychiatrist.
Personally I wasn't impressed with the taster day as the impression I was given by the guy running it was that you are basically there to help the person do their art more than actually try and cause any change... and most of the work was in prisons according to him as well. No idea if this was accurate but it's what I was told.