I went to a Taster Day with TEFL Lab. It was a day course and cost me around £59 (which they deduct off of the main TESOL course if you decide to train with them). They also, offer a weekend introduction course as well and because I was the only one to attend that particular TEFL Taster Day, the teacher pretty much did the weekend introduction for me instead meaning that I got more for my money. You can find out more here
There was also, a second school called Training 4 TEFL who I was going to apply with. It's kinda ironic because initially, they didn't have a Taster Day. I remember calling them about it. Then about two weeks later after I did my Taster with TEFLlab, they're now looking to introduce one. xP You can find out more here
As I only work part time, I was initially looking for something cheap. I even found a college that did a Taster day for £40 and another one that did it for a tenner, but what I found was that they didn't provide me with as much information via email/telephone correspondence, and there was no defined structure for the day. In my opinion, I think colleges (those that specialise in a variety of different subjects e.g. Golders Green College) might be somewhat lacking so don't be afraid to spend a bit of money ( - they might even deduct it off of the total of the 4-week TEFL course). I'd recommend an English Language School or schools dedicated to training English teachers for these. I found mine through the Trinity College website (as the Cert. TESOL is their qualification). You could probably do the same with TEFL schools that cater to the CELTA qualification too.
Hope this was somewhat helpful. =)