Symonds only requires students to have an A* or A at GCSE for Maths and the Sciences - because the evidence suggests that students starting A level courses with any lower will probably be wasting their time, unless, of course, they can match teaching time with tutored time, etc.
In MFL students need to have a B.
For most other courses the requirements are the basic entry requirements of five A*-C passes, including Maths and English. This is not selective.
Some courses require, e.g. a good pass in English (B) to do History, but this part is not always enforced.
Students who miss on their offer are usually offered a L2 course with resits in Maths /English, whence they can progress to L3 the following year. Students who only just meet the entry requirements are guided towards three rather than four subjects.
This is not very selective. In the sector, PSC is regarded as an open-access college.
Nowhere offering A levels will accept any student with low GCSE grades - most schools expect on average a grade, or even two grades, higher than than PSC ask for. No college would allow a 16 year old with D or E grades to start an A level course - and I don't think that any that do are doing the best thing by those students.