It depends on many things ,including the quality of university you would be going to. For example, at the extreme end of the spectrum, if you could get a place doing computer science at somewhere like (eg) Cambridge or Imperial then you would probably have far better long-term job prospects that you would have with 3-4 years of experience. The same applies to the other Russell Group/1994 universities, and probably also (to a slightly lesser extent) to the ones just outside that. But if the university you were going to go to is very low-ranked, then its debatable whether a degree would give you more prospects than several years of (good) experience and maybe some certifications. As long is the job pays reasonably well (>£18-20k say) and has good career advancement prospects then it might be worthwhile doing that instead. But it really does depend on your situation.
As well as the job prospects, university can also expose you to more diverse areas of computer science, both through classes, and from the fact that you have a lot of spare time to work on your own projects. So while taking the job would let you learn about IT issues such as networks and sysadminning, a degree might introduce you to areas like machine learning, bioinformatics, software engineering, 3d graphics, etc (in other words, you would get more breadth but less depth). Its possible to learn all that stuff on your own of course, but balancing self-study with full-time work is hard for some people. So if you think you might want to do something in the future other than IT work/web programming/etc, university might be something worth considering.
But whatever you do, you arent necessarily committing yourself for life; if you take the job and decide you want to go to university instead when you're 21 or whatever then that may still be possible, and if you go to university it may be possible to get summer work in IT.