There are very few jobs where a conviction or caution automatically bars someone from employment. (To the best of my knowledge, the only example in the education sector is that nobody with a conviction, or caution, for a sexual offence against a child is permitted to work in a school, irrespective of the age or sex of the children in that school). Otherwise, it's entirely at an employer's discretion as to whether they regard a conviction, or caution, as a reason for refusing employment.
FE colleges normally accept students from the age of 16, so all employees will be required to undergo enhanced CRB checks, irrespective of the ages of the students that they'll directly come into contact with. For the purposes of such a check, convictions are never regarded as spent. (Cautions can't officially become 'spent' anyway but they'll still show up on a CRB check).
Your problem is the litigious nature of modern society. Employers are extremely careful to avoid any possibility of legal action against themselves. If a college's board of governors saw your application for a job, they'd have it in the back of their minds that you might get their students involved with drugs. Worrying as that would be, they'd be even more concerned that they could then be sued for their negligence in appointing someone with a known connection to drugs.
In theory, your caution doesn't prevent you from obtaining employment within FE. In practice, it almost certainly does.
Chris