As above, engineering tends to have a lot of contact hours compared to other subjects in first year (and later on, just it's particularly bad in first year), but you are largely free from having to do other work which means you can probably hold down a weekend job if you want to. A lot of universities will offer work on and off which is another avenue of making money, it just won't be as regular and you won't make as much overall (usually pays quite well per hour though because a lot of unis are living wage employers and you get holiday pay in lieu).
I'd disagree that you need FM though, most unis will start teaching maths from the relevant parts of A2 maths plus a little bit of AS FM which you need thrown in, you don't really need anything more than that throughout your studies, with a few exceptions. There might be some unis out there which go overboard with the maths content though.