No, you have to declare all qualifications. If you don't and your university finds out later, they can remove you from the course, regardless of whether you've already started or not.
is it possible to not show universities a specific grade you got at gcses , say u got 8 a*'s but one e is it possible to not show that e.
You can try it but some universities check and I have known Liverpool, to name one, reject a candidate on the grounds of dishonesty/loss of trust for not declaring a C at GCSE PE.
but like what if you orginially planned to drop the course but your school refused
You will still have to declare it. But if that's the case i would recommend emailing the individual university admissions tutors and course directors to explain your situation so that that they will take it into account when reviewing your application.