Many mature applicants take GCE A and AS levels, Scottish Highers, BTEC National or other SQA qualifications, but in exceptional circumstances, some conservatoires may accept prior experience or learning instead of formal qualifications. You should check each conservatoire's individual policy directly with them. The normal entry requirements for UK courses are provided in Course Search. Some conservatoires may express entry requirements using UCAS Tariff points. Visit the UCAS website for more information about the Tariff.
If you have not been in formal education for some years, you may want to consider taking an Access course at a local college to brush up your study skills. If you left school with few or no qualifications, this will help you to prepare for your return to study and get entry qualifications for your course. Please visit the Access to HE website for further information and to use the course search facility.
When applying for a conservatoire, you will need to provide evidence of your ability to study at an appropriate level, including musical ability, or evidence of relevant experience. Exact requirements will vary between conservatoires. 
For this reason, it is a good idea to make direct contact with the admissions office at the conservatoire in which you are interested to discuss your position and the options available to you before making a formal application. You may need to provide a detailed curriculum vitae - ie a written listing of your educational achievements and your employment history. Qualifications taken several years ago - for example, the Certificate in Secondary Education (CSE) or GCE 'O' level - will still be taken into account.
It is useful to be aware of the competition you may face when applying to conservatoires. Whatever your age or qualifications, it is more difficult to obtain a place on a popular course.