Here's where we're trying to give you every test or relevant past exam paper we can either find, or give you an outside link to. When you get this far (say around April or May) then it'll be up to you to work through as many papers relevant to the board you are studying for, to get you through those exams with the highest possible grade. We know you can do it!
The good people at Maths Made Easy have made a lot of the past papers available on their site for each of the exam boards:
AQA
For AQA, the past papers for the current specification, and past papers from the previous specification that are still relevant to students taking the current spec. can be found here: mathsacademy - AQA. Get through that lot, and you should be fit for anything. Well, almost...
EDEXCEL
If you're taking the EDEXCEL exams, you'll find past papers for their current spec. at: mathsacademy - EDEXCEL. Get through that lot, and you should be ready for the exam. Well, almost...
OCR
Past papers for J277 (current spec. - if you're doing the OCR GCSE this year, this is the one for you!), and a few from J276 (previous spec. - when there was still an examined project) that are still relevant to J277 students. Check it out at mathsacademy - OCR. Get through that lot, and you should be ready for anything. Well, almost...
WJEC / EDUQAS
If you're taking your GCSE with WJEC / EDUQAS (The Welsh board) there are past papers from their current and past specification at mathsacademy - WJEC. Get yourself through those, nothing (much) in the real exam should faze or throw you...
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Other subjects...
You might just find some useful stuff on that site for English, Mathematics and Science, too, if you dig about; just be sure it's for the syllabus you are taking!