The first thing to grasp it that CTC and a number of others are only after your money, they charge you a lot to get the ATPL and then take a rake off from the airline they place you with.
The selection is all about making sure they can that they can get you through the training in the minimum time as this gives them the best return on their money.
Flying before going to CTC or the like is just the same as handing them money as you are going to decrease the amount of work they have to put into you.
Having had some dealings with them I have no doubt that they will tick all the boxes, however there is something that I found slightly unsettleing about the place, a mate of mine discribed it as aviations answer to the church of scientology.
If I was doing the ATPL thing for a second time I think I would look as Stapleford or Bristol filght centre, both places have good reputations and will cost you a lot less.