For engineering Imperial soars far ahead of UCL, it's not even a debate. Given that Imperial is first and foremost an Engineering school, and for UCL engineering is pushed to the back somewhat, I imagine you will also find a lot more related activities at Imperial.
In terms of lecture hours, that's not really something you should look at in order to determine academic rigour as it will be a red herring. UK universities have a strong focus on independent learning so the number of lecture hours won't make a whole lot of difference, and for topics where labs aren't appropriate you almost certainly won't need contact time. In fact you could make a decent argument that more lecture hours is the sign of a worse course. In first year you will see 30-40 hours of contact time per week no matter where you go due to a combination of lots of smaller labs, transitioning you to the uni style of teaching and learning, and bridging gaps between A levels and engineering. In 3rd or 4th year you will likely have about 10 hours no matter where you go, because you're expected to be independent, you're expected to be able to keep up with the fast pace of lectures or read up if you can't, you're expected to do projects outside of your course and other extracurriculars.