HNC in computer platforms/software engineering covers very basic web design. You get to learn a little bit about web technologies/languages and do a web project which can count as part of your final marks (your final result depends on compiling a number of the best module marks you gain throughout the course).
If the HNC is in computer platforms/software engineering then it certainly isn't geared towards teaching web development/design as its main focus.
That said, you can use the skills you will learn to help you along the way and, if you choose to, you can opt to do a web based project as your chosen final software project.
The way you evaluate requirements, plan, document, write, test and otherwise demonstrate good software engineering practice will all be key to your final result, but there is nothing stopping you from going that way if you choose.
There is quite a bit of flexibility in the HNC computer platforms/software engineering course and although some modules and topics are a must, you can always decide for yourself what to do for your final, and if you prefer the web then you can use this as an excuse to build on web skills. Your tutors should be happy to help where needed.
Essentially you will get an overall grounding in networking including topography, servers, switches, security layers, server operating systems (a good HNC course will include Windows and Linux) and you will also spend a lot of time on Visual Basic and good database design including data normalisation, mostly using Microsoft Access.
You should find your tutors more than willing to help if you start asking web based questions, but maybe not if it's in the middle of a Linux server lecture...
I used to teach HNC/HND HNC in computer platforms/software engineering for my sins, and the first lecture of a new intake with me would begin with setting up dual booting Windows/Linux computers. It went downhill from there...
Good luck with things no matter which way you go with it.