I'm too old for such things
But my daughter is just coming towards the end of the first year of her Phd (in Chemistry) at a top UK Uni (Durham) - in order to gain such a place you have to have excellent prior qualifications (she has a First Class Masters Degree, from York - another top Uni), a good CV (she has an excellent CV, including a year working in Industry in the Netherlands), and good references (never seen them, but her professor at York and employer in the Netherlands both gave her excellent references - and the girl who showed her round at Durham commented "So you're the girl with the 'references'"). Along with all that you have to be interviewed, sell yourself at interview, and then finally offered a place.
So I'm very familiar with the requirements for Post Graduate places
Starting off saying you're "an average student" isn't a good idea, only exceptional students are likely to win Post Graduate places.