You may apply for student visitor visa for this purpose. Another option for this is if you contact the UKBA directly and ask them for their advice , they may be able to guide you for right visa.
If the brush up is less than 6 months, then of course you qualify for the student visitor regardless of how many hrs/week. But if you think that you may need to stay in the UK for longer period than 6 months, or perhaps you need to go back before 6 months and then come back again for second brush up etc, then you may fall under Tier 4 General (student) category.
Again, if you contact UKBA , they may be able to guide you much better.