Most of your questions will be answered differently depending on the academic field.
I can't answer the funding questions, as my experiences are from my own university days, pre-student loans.
A second degree need not be postgraduate - a degree is postgraduate only if you need to have done an undergraduate (first) degree in order to apply. So, say, an MA course will almost always specify that you need to have done an undergraduate degree (say a BA, or BSc) in an appropriate area to be eligible to apply. But it is possible to sit a second undergraduate degree, though it implies someone is changing direction, or doing it for fun. (I have a DPhil in English, but would love to do a degree in art history for fun - this would be a second undergraduate degree for me, as I have a BA in English already.)
A PhD is the ultimate substantial research postgraduate degree degree, where you engage in three or four years of research and write up your results as a book. Not sure what you mean by what it's needed for, but mine was necessary for my job - I'm an academic.
Taught degrees are comprised mostly of classes/lectures/seminars examined by essays and/or exams. Research degrees (PhDs, some other postgrad degrees) are much more about independent, supervised research, with the resulting thesis/dissertation examined, sometimes by a viva vice, or oral exam. When I did an MA (early 90s, not UK), it was two taught terms, then four of writing a 40000 word dissertation - now most MAs are taught courses with shorter dissertations.