Did Oxford say that their modular course is not a controlled environment? If it isn't controlled at a school where all is done in one continuous course (which it can be at Oxford) then you certainly should not attend that school for any course. What they are trying to do is make up reasons for people to pay for an integrated course. Actually both their courses have a good reputation, but I can see no good reason that the integrated course is superior to the modular.
Hey, bung me £5000 and I'll control the environment, by sticking you on appropriate courses, onto each as you finish the last. Each stage is then controlled, and I am happy to push you to the next stage in exchange for the next tranche of my outrageous fee. You'll save £20,000.
Alternatively go to a different school (I know of at least two who aren't trying to sell integrated courses) which will give you a whole set of modular courses, zero to fATPL for far less money than that, and stream you through those courses in a controlled way.