How Many Years I can study under degree Level?

Hi

I had one student visa + 2 extensions for ACCA course:-

Student visa - 1 year [level 6 PBS]
1st Extension - 1 year [Level 6 PBS]
2nd Extension - 1 year + 4 months [Level 7 Tier 4]

Now at present in home country. returned before 2nd extension expired. Planning to apply for different course as unable to show progress in ACCA.

During level 6 cleared 6 papers. In level 7 cleared nil papers

I want to know if I have not completed level 6 course and then got shifted to a level 7 course and not cleared single paper can I switch over to a different level 6 course? Will UKBA ignore my having attended level 7 course on basis I didnt clear a single paper? Also I am confused about the degree level cap? I have totally attended 3 years + 4 months of which 2 years in level 6 and 1 year + 4 months under level 7. So how many more years can I study under a degree level?