"I was put off by people at school - my cabbage wasn't as good as other people's, you know, so that put me off"

In this quote, Roger McGough is sharing that his lack of ability or success in a certain location (in this situation, possibly expanding cabbage) triggered him to feel discouraged by other people's talents or accomplishments because very same area. This unfavorable experience at institution may have led him to avoid seeking that passion further.