Regarding the smart wear, I think you should speak to your form tutor or head of year about this is: if you truly cannot afford anything, not even from a charity shop or the like, they may let you come in with the clothes you already own and the school may even have a stash of spare clothes, you never know.
Also, why are you late to school to every day? If it's due to you getting up late then that's something you just need to sort out, set an alarm for earlier, go to sleep earlier too so that you're properly rested.
And as you say, you recognise the low grades are due to your poor work ethic and non-existent revision. This is something you need to fix too, how do you know you can't revise if you've never done it? Bear in mind you don't need to revise many hours each day, if you do a little bit consistently then that's a start because there's no way you're going to get your grades up without revision. And if you go to uni you're going to need to revise, so better to start that habit now rather than later when the material will be even more difficult.