I think the cost difference that you are referring to is actually this... there are 2 ways to obtain your frozen ATPL, the "Modular Route" and the "Integrated Route".
Modular means just that, you can do your training in chunks whilst perhaps having gaps in-between to continue working, etc. This is usually much cheaper (20-30K) and you can go to different schools to do each module if you wish.
Integrated means that you fork-out much more (30-60K) to attend one school full-time. It has been suggested that having completed an integrated course that you will find it easier to gain employment, as apposed to modular, (this has been fiercely debated topic on PPRuNe).
I personally am following the modular route as I could never afford to do an integrated course.
If you look back through the many PPRuNe threads you will get a really good grasp of what you need to know, and will be able to make a well-informed decision as to which route is best for you.
By the way, OATS is one of the places where BA send their cadets, WMU is too, and I think perhaps BAE in Jerez is another, but I may be incorrect on that one.
All the best,
PPP