If you have not already done so, contact the university's NUS office. They may have someone there who can help lodge an academic appeal to find an acceptable solution. Did you have any mitigating circumstances? It seems a little strange that there is no mechanism to allow a retake of final year module. There is a Cass business school person on here, perhaps you could PM her to see if there are any other options for the OP.
The OP's problem may be that, many places will not allow you to transfer to complete just the outstanding credit or just the final year and award you their degree. Some might ask you to complete the second year as well as the third year. You would more than likely not be able re-use your dissertation credit if you are allowed to complete the final year at another institution. Despite having the CATS, transfers are very much at the discretion of the receiving university.
Depending on what your subject is, you could look for places that offer a 1 year BA(Hons) top up course, these are designed for people who have at least 240 credits or a HND, the top up year which includes a dissertation is 120 credits. They less likely to have the reputation of Cass, but you would get an honours degree. You could try Birkbeck, I know they have in the past accepted OU students into later years of a degree program, but not necessarily the final year.
Alternatively you could apply to an MSc or MA, you would need to explain why you were awarded an ordinary degree. But this won't help if for job purposes you need an honours degree.