There are no specific educational requirements to become a police officer, so the best advice is to take subjects you enjoy, that will develop you best as a person mentally and intellectually and that will make you a more rounded person.
University or a college course is definitely not necessary - in fact, they're often a waste of time and sometimes not even endorsed by the local constabulary. If you'll be leaving school mid-2019 or after 2020 then yes, the new entrance policy will apply. There will be a number of options, including a self-funded degree for three years followed by finding a job in a force yourself, as well as the more appealing three-year "degree apprenticeship" with a job at the end, or a one-year conversion course if you're already a graduate.
If you're looking to join before then, there are no educational requirements as standard, although you will have to prove your capabilities in basic English and Maths, and some forces will ask for a minimum 5 GCSEs.
Whatever route you try and take, if there are no vacancies in your chosen force at the time you apply and you don't wish to move to another area, then your best choice is to take up a volunteer role in the Special Constabulary of your local force. You'll be a fully qualified police constable with the same powers of arrest as any other constable, and it will give you valuable experience. Many special constables go on to become full-time regular officers, and are often considered before those outside the force. You could, of course, take up a role as 'police staff', such as a case investigator dealing with the investigation of minor crimes (interviewing suspects, etc), or a detention officer in a custody suite, or even choose to become a Police Community Support Officer. These are paid roles, and can put your 'foot in the door', so to speak, but don't come with the same powers and responsibilities of a Special Constable.
Whatever you choose, the police is a great career. Hope you do well!