To teach in the UK and overseas you need to get at a minimum the CETLA or what used to be the CETFLA (Certificate of Teaching English as a Foreign Language to Adults). There are two certificates one is from Cambridge and one is the Trinity. Don't be fooled by on-line courses, weekend courses or anything else. These are the two to get you qualified. Both are intensive theory classed followed by in class teaching and observation.
I went to International House in Barcelona for my training and followed up by teaching in Poland with them.
http://www.ihworld.com/You normally are expected to have a university degree in something first.
The other route is to add EFL/ESL qualification to an existing education degree or to persue a Masters in EFL/ESL.
I currently am working teaching teens and adults both privately and through a UK organisation so I am more than willing to answer any questions you have. You can pm me or post here.
While I teaching UK English we have lots of fun discussing the differences and most people are happy to have access to an American teacher without having to pay to go to the USA. :-)
Also try TEFLA instead of TEFAL!
Also see:
http://www.eslcafe.com/