No, 120 credits is the maximum and only number of credits you could take. The University does advise eating and sleeping ;-) and tbh if you really want to fill your time with study and nothing else, you can read further around a subject and do more towards your modules.
To the best of my knowledge, yes those 30 credits could be in anything. However, PPE is a new one to me as it wasn't in existence when I was involved with the degree so do check.
However, do think before you decide on them. A language would be an ideal one to do, however, with something like science, for 30 credits, you really wouldn't be getting the breadth needed, and you will find that come semester 2 or year 2/3 etc. you won't have done the module needed in order to take much in the way of science. It gets a bit trickier in second/third year but that's something you will understand more when you get to that point.