How do journeys within a website feel, how does functionality feel? Does it complement the rest of the site, the journeys and the intentions? This is what user journeys are all about.
UI or user interface is all about the design and it is as critical a part of the user experience. Depending on the complexity of your site, your users journey and your user demands, the requirement of UI & UX can quickly increase.
If you are looking at a site from the top down then UI is critical to things like the styling and consistency and attention to detail whilst the UX focuses on the points that will make your customer convert.
Whilst the two go hand in hand the easiest way to detail the difference between the two is to say:
A UI will deliver 5 beautifully designed pages that deliver on brand guidelines and are aesthetically pleasing to customers. A UX will take this and tell you that you can achieve the same result with two pages.
While UI is looking at design, UX looks at function. The two are very much a couple in the stages of website development and successful delivery.