Knowing your subject inside out is one thing, but the skill to teach that subject is something completely different. Not only will a teaching qualification help you to develop this skill, it will also help you with regards insurance and accreditation. Very few insurance companies will offer you teaching cover if you are not qualified, and accreditation of a course also usually requires that you hold a teaching qualification.
There are a number of teaching qualifications around, such as PTLLS and City & Guilds. Your local college should be able to give you some information on these.