"Top 15" is really wide margin. In any event I'd expect the workload to correlate to that of any other degree at that uni. However I'd expect that to be different from a "top 3" uni to "top 15-10" uni. Also as noted above, quite a few of those "top 15" unis may have specifically poorer than average engineering departments (UCL seems to be mentioned a lot in this regard - also from personal experience I can attest Exeter is in this category, which even some academics there agree with), and there may be other unis outside that which punch well above their weight for engineering (e.g. Loughborough and Strathclyde are very well regarded for engineering I gather). Try not to limit your perspective too much in that regard.
Beyond that, it's a degree so the conceptual difficulty will be somewhat higher, and unlike at A-level you won't be "spoon fed" the material - you'll have lectures giving you the basic outline and/or going through some examples but you'll be expected to do the bulk of the learning and practicing outside of lectures in your independent study time. This is true of any degree - undergraduate study is just different to A-level study.
However engineering is much less conceptually difficult than other degree subjects though, in my opinion. Compared to say, maths or philosophy or linguistics, it's pretty concrete and easy to practice worked examples in. There isn't much abstract material that can be hard to grasp - it's just a case of grinding through stuff and practicing to make sure you understand. So that may make it easier to initially approach material, at least, for a wider range of students. Of course equally it does mean you may just need to spend more time doing practice problems etc to "drill" the material in.