Hi I applied to KEGS for 2020 entry but failed and I currently continue in the same school I went to before 6th form (WHSB). If you have all the other entry requirements, you should be fine. KEGS need a 6 in Maths and both the English subjects (I'm assuming as this was all I lacked with a top 8 of 71 (1 less than the possible max) - I got 5 in Eng Lit and 6 in Eng Lang). They also need you to pass a foreign language (I don't remember if it needs to be a modern one, meaning French, Spanish, etc as opposed to something like Latin). If you pass all of this, you should have a pretty good chance.
You also need at least a 7 in all the subjects you plan to continue with an 8 (preferably 9) in maths to do FM (probably shouldn't be a cause of concern for you)