You won't be able to do that. The point of Distance learning is that it is a modular course which is monitored by a training school. In order for the school to authorise your exam entry (you can not just turn up and do the exams) the school has to verify to the CAA that you have not only completed the DL course, but also attended a minimum number of hours in the classroom.
If you attend the course at CATS, Bristol will not authorise your form, and CATS can't authorise it as you are not enrolled with them.
Therefore, if you want to do the CATS brush up, you will need to reenrol and do the full CATS course with them, otherwise, negotiate what you want with Bristol.