My advice is: don't get too hung up on the module choices you make now. I've just done my module registration for 3rd year CS, but they are by no-means concrete. What me and my fellow CS students tend to do is to register for everything and then deregister later in the year before the exams. You get several oipportuinties to change modules throughout the year and can de-register at any time up until april (i think).
Also, are you sure you have clashes, this is very very rare. If your basing this from your timetable, look at wether its a seminar (which there are often multiple sessions of in a single week) or a lecture.
Computer Security is an amazing first year module, i believe mathematical programming is quite ok, but A Level Maths Discrete Maths module is useful for it. They really don't give you much choice in optional modules this year, we had principles of concurrency back in my day! When choosing your modules, bear in mind that you can take modules that aren't on the list from other departments (i took Stats from the Maths dept, for example, which used to be on the main list). I also suggest thinking about doing more than 120 CATS, because it reduces the grade boundaries and doesn't add that much extra work in your first year.
As for essay writing, plenty of that in the Professional Skills module. Sorry I couldn't help you precisely about Elements of Scientific method, just register, go along and if you don't like it, deregister!