I'm not sure where I stand on legally mandated helmets for bike riders. In Australia - or at least New South Wales - they are compulsory, but in the UK they are not. I wear one pretty much always anyway, except for the occasional short shopping trip. My bike lock will hold my helmet, so I don't have to carry it around.
Lights, on the other hand, really are essential at night. The Oxford police could make a fortune enforcing the law on this - it's apparently a £30 fine - as an awful lot of cyclists blithely ride around nearly invisible in the dark. (Though fewer now than two months ago: either Darwinian selection at work or the first years have got a clue!)
I just have ordinary three-LED front and back lights (I haven't gone bike light crazy), but I'm using rechargeable batteries with a regular recharge schedule, so I don't spend half the time with dim lights. I also have a fluoro yellow rainjacket.
I always keep an eye out for how visible other cyclists are, both when cycling myself and when driving. I suspect most of the students cycling without lights don't drive themselves. And Oxford drivers are probably so good around cyclists because they are mostly cyclists themselves. (I certainly feel a lot more comfortable around bicycles now.)