You do need a PPL before the official start of your course. Of course no-one can stop you brushing up on the books before that, and it will help your PPL exams - in fact if you work really hard at these, and take the broadest interpretation of the requirements it will be a help towards your ATPLs.
You can certainly carry on swimming. The CAA recommendation is 15 hours study per week, though more than this is perfectly acceptable. You are supposed to do 650 hours of study - 65 must be in the classroom, but most courses give around 120 hours of classroom study.
Most schools split the course into two modules, each allowing you to sit 7 exams of the 14. Different schools split the subjects differently, but that is not to important as they are carefully thought out to complement each other.
You will be expected to work at home for a few months, testing yourself with worksheets, occasionally an assessed paper to send back to the school so they can monitor your progress. Then you choose a date for a set of exams (first full week of any month) and book a two-week brush up course. Here your instructors will run through the syllabus as they see fit and put more tests in front of you. This bit is hard work (as if it isn't all!). Sit the exams, pass with flying colours and repeat for the second half, or pass some and decide whether to resit before starting the second half.
Any more detailed questions feel free to contact me by private message (I work for a school).
Best of luck. Don't bump into the end of a swimming pool (my mother had a stroke that way, would not be good for a flying career!)