The "new" grading system in UK schools make no sense to me at all, as I was brought up under the old system.
Basically you're looking at aiming for the equivalent of the old A* to C grades, any lower and colleges for Post-16 courses or Apprenticeship schemes don't want to know (look them up for yourself and you'll see).
To get better at it.... first off, start investing in books... in particular the self-help books everyone else refers to after they've left school and found going there a fat lot of help or they just kind of forgot after not putting any skills they did learn in school to use for a lengthy period of time.
The "for Dummies" + "teach yourself..." ranges of books are particularly good, as well as the "Need to know?" + "Gem" series of books from Collins publishing, and a handful of books from Haynes publishing (generally better known for their car repair manuals, but they've expanded into other topics of book).
Of particular use for improving your maths I suggest looking at the books:
* Basic Maths for Dummies (ASIN: 1119974526) available as Paperback or a Amazon Kindle ebook
* Mathematics: A Complete Introduction - Teach Yourself (ASIN: B004M18WMG)
I always found schools not that great at teaching maths in a way that sinks in - they'll tell you how to do it, and/or give you some sums to do either off the board or from a workbook / sheet, but never give you any text books to go back over how you're mean't to do it as many times as you need at your own pace until you've got it.
For improving English:
- English Grammar for Dummies (ASIN: B002W83E7E)
- Collins Easy Learning Grammar & Punctuation (ASIN: B005IH026C)
- Collins Gem: Science Basic Facts (ASIN: B00GQV14DO)
- Chemistry for Dummies (ASIN: B01GCD5BSY)
- Biology for Dummies (ASIN: B06XGS92CS)
- Bad Science (ASIN: B002RI9ORI)
- Bad Pharma (ASIN: B008PCVGKI)
- Bad Astronomy (ASIN: B0014ESXVY)
- Patrick Moore's Astronomy: Complete Introduction (ASIN: B0106471RY)
General Practical Skills + Lifeskills, etc
- Trick and Treat: how 'healthy eating' is making us ill (ASIN: B00APDUOQC)
- The Meat Fix (ASIN: B006YZ27LU)
- Eat Meat and Stop Jogging (ASIN: B00JBNPV06)
- SAS Survival Handbook (ASIN: 0007595867)
- Haynes Job Hunting Handbook (ASIN: B00MHUSVIY)
Also a selection of YouTube channels that are handy for learning stuff:
- The BackyardScientist
- Scotty Kilmer
- Linus Tech Tips
- Project Farm
- This Old House
- Hydraulic Press Channel
- Beyond the Press
- The Samurai Carpenter
- Andy Rawls