I don't think GCSEs would be considered choosing specialisations, though they are moving in that direction. You still have to learn English, maths, and science, but you can get rid of things you don't find valuable (for me it was music, drama, stuff like that) and keep what you do want (for me that was geography & history). You also have to do physical education and (in some schools, at least) religious education.
Alongside your compulsory subjects, you have four choices. Some schools, depending on their specialisations, may make it compulsory to have a choice that is in a specific area (e.g. a design subject, a performing art, a language). You also have the option to do extra, more advanced science. It's not really a specialisation, as you can't really go into anything like construction, but it's the last two years of compulsory education for everyone and it lets you focus on stuff you like more and remove what you don't want.
In most schools (I think, anyway; mine was different) you finish high school with your two GCSE years (ages 14-16) in highschool, which is secondary education. You do your A-levels in colleges for two years (again, this is different in different places, as I did my GCSEs and A-levels in the same college. It's when you do your A-levels that you really specialise.
At the end of your GCSEs, if you want to continue in your education, you pick your A-levels. You will pick 4 subjects, also quite typical subjects (e.g. Biology, Geography, English language, French, Maths, Graphics). At this point it's mostly academic, but you can do more practical A-levels. Mostly though if you're doing something like construction you'll leave school and get an apprenticeship or something. You can do more, but four is the typical number. At the end of your first year you will have completed your AS-level of your chosen subjects and most people will drop one and finish their last year with A2-levels in their subjects. They're graded A*-E (excluding fails) and it's these grades that are the greatest factor in which unis accept you, which is where I would consider where you specialise.
I hope I made that clearer and I will try to explain better if I didn't. I followed an academic path through my GCSEs and A-levels so I probably haven't told the full story of how it works as I only really thought about my perspective. I also did my GCSEs about 5-6 years ago and my A-levels 3-2 years ago so there will probably have been slight changes.