Heya, I'm starting Graphic design at lboro this September and will say portfolio making was THE MOST stressful thing ever but don't let it put you down!!!
What I've found all of them look for is process. Genuinely the most important thing they want to see is not the final outcome but how you developed your ideas, and even better if you already do a creative subject in sixth form/ college.
Include everything from initial sketches to even failed attempts and how you managed to rectify and recover the piece. They definitely want to see that you are learning by making mistakes along the way and that you are able to pull yourself out of those problems.
Another thing I would recommend that I have done is find something your're i terested in and make a project out if it because anither thing they want to see is your passion for design and how you can use inspiration from things around you. For me i did a couple of poster making projects related to my culture and to some lyrics of songs i liked as well as a project on greek mythology sketches.
Graphic design is basically a way to communicate ideas and thoughts visually so the more youre able to add in symbolism the better!
One thing I wasn't prepared for but just be ready for is that some unis will ask you to complete extra tasks. I know for UWE we had to prepare for 5 tasks and Loughborough always always prepares a design task for you where they give you a prompt and you have to cready any piece of design as a response.
It's good to have some brief based projects but personal briefs are infinitely better because they represent You and thats what they want to see.
But nevertheless PROCESS PROCESS PROCESS!
All unis adore this so bring focus to it!