jechać, pojechać and jeździć
jechać - be in the process of going (by vehicle) - so imperfective
pojechać - to get somewhere (by vehicle) - perfective (you see the po added to jechać so, any other preposition added would also yield a perfective (like zajechać, dojechać, najechać)
jeździć - this is the frequentative
of jechać (meaning to go from time to time, to go regularly or repeatedly) - frequentatives of any verbs (be it perfective or imperfective) are imperfective (buuuuuuut a perfective form can be created of many frequentative forms by again simply adding a preposition (typically po- is a semantically neutrall preposition))
Jechał do Londynu. He was going to London. (was on his way)
Pojechał do Londynu. He went to London.
however when talking about future this is not so clearcut and you can say Jutro jadę do Dublina. I am going to Dublin tomorrow. Jutro pojadę do Dublina. is sort of over the top (although Jutro jadę is rather colloquial and Jutro pojadę is literary)