I recently started looking in to this. It is slightly overwhelming, but something I would really like to do over the next few years.
I too have been looking at the training through the British Wheel of Yoga. This seems to be the most regulated and widely accepted qualification in the UK. It does take a bit of time, however. The course at a studio in my area requires a foundation course (6 months--1 full day per month for a total of 60 hours) and then the teacher training diploma course which runs over 2 1/2 years (1 full day every 2-3 weeks). The entry requirements for the foundation course is generally 2 years of yoga practice. I haven't enquired about whether the 2 years should have been done at this particular studio or not. I mostly do yoga at the gym (really good teacher at my gym, and haven't had the money to join both the gym and a yoga studio) so I don't know if they will look down on that.
It seems to me that the situation in the UK is slightly different than that of the US, where there are lots of different certification options, some which are significantly more involved than others, although I could be wrong. I know my friend managed to get certified to teach yoga in a matter of a couple of months. I have no idea what her training was like. I also believe that this is something that is starting to change. I just saw this article in the NY Times a few weeks ago:
http://www.nytimes.com/2009/07/11/nyregion/11yoga.htmlI still have a lot of practicing to go before I would be ready to sign up. Recently I have been able to get to class 3-4 times a week, which is great, but I am trying to work on doing a daily practice at home.