I feel like the IGCSE times were more than enough exposure to those other subjects to give me a reasonably well-rounded image of those subjects. Now, a decade later, I'm definitely glad I went the STEM route rather than ever touching on any humanities subjects in any amount of detail.
British universities do often (usually?) require English language (or an equivalent) and maths (I)GCSEs, as do British employers and further education colleges etc. so its a really bad idea to skip those.
I do think its good to give kids a broad education, and for some it would be good to continue that for longer. For others (like you) its just boring.
I also think there should be more opportunities for adult education, so if you decide later in life that you would like to learn about English literature or physics or whatever you should have a chance to do it. AT the moment in the UK things seem to be going the other way, with an over expansion of universities sucking up money, and further education colleges that used to offer adult education putting the resources into "16 to 18" courses.
But yeah, until that age it sucks and a lot of people struggle because they have to do classes they aren't at all interested in.
You usually need to pass English and maths GCSEs, but universities mostly care about the subjects you do in the last two years (except for very competitive courses).
This can be a problem for those who want to keep options open until they are 18 (like my younger daughter).
Even the system up to 16 is pretty flexible. There is a huge range of available subjects - although most schools offer only a limited selection (my kids were out of school by secondary school age so we had a huge choice and did some less usual subjects like astronomy and Latin).