Hi, I'm a 1st year so I'm only talking with 4 weeks experience but here it goes. There are about 12 hours of lectures per week so far but that doesn't include labs (which aren't mandatory but are I think 6 hours a week and you'll probably have to go to at least one of them) and tutorials (about 5 hours a week of which only small group tutorials are mandatory). Now, in terms of the workload, so far it hasn't been anything insane we have a programming, a logic and a maths methods assignment every week so far, which is actually a good thing because it ensures you don't get behind and that you get feedback on your mistakes. Other than that there's unassessed exercises which you can do for practice as you're studying but at your own pace. Most courses advance pretty fast but if you set about 1 or 2 hours per module a week you'll be fine I think (except for programming with which if you don't have experience you'll need a lot of time). So far I don't know how hard it is to get a 1st but if you study enough you definitely can.
Also, didn't do GCSEs or A levels so can't respond to that.
Most lectures are recorded (not all of them though), however missing many lectures is not a good idea. Hope I helped