There are lots about but it can be bit of a minefield.
I'm not a fan of fast track if your looking to go self employed.
I did all my tickets together about a year ago but after years as a plumber, working with and arround qualified gas guys.
I still find it hard work to keep on top of my game. I think its dangerous to go straight out on your own after a few weeks intensive training.
If your set on going for it id look seriously at finding an experienced guy to do some work with after you qualify as this will certainly be the most difficult to aquire.
ACS runs through several training bodies who in turn licence training providers. I used a construction skils acredited center in milton keynes and it was good, if you search their site youll find a training center in your area.
Then you'd best work out what tickets you'll want (stick to min of what you need) you can add others later.
do you have plumbing experience? would a basic skills based course be of use?
you may need water regs ticket, unvented etc.
I rang round and asked questions.
The most helpfull and consistant center got my vote!
good luck!