Talk to your school about it. They may not be happy, but they will help you, is the first thing i would say.
The second one is to ask for help from family/friends for filming - if you make plans with them, you won't cancel/let them down - commit yourself to filming with them.
Finally, have a look at the 'individual' sports that are options that you can do for your PE GCSE. The options differ by which board you are sitting, I believe, so keep that in mind. Individual activities will allow you to do filming without relying on a team to be there - more flexibility.
Hope that helps!