Hi I can't help you with providers but just a few tips.
Contact local exam centres/6th forms/colleges ASAP to double-check where you can actually sit the exams you need to as an external candidate.*
If you are doing your A-Levels with the intention to go onto a degree, e-mail the course providers you are interested in to see what their policy on the Practical Element is - your Practical Element isn't graded, just a complete/incomplete status. Some science degrees don't consider the Practical Element necessary for admission (though you still need to explore the theories behind the practicals, as they will come up on your exams and aren't taught formally), but others (such as Medicine) do need you to complete the Practical Element. And to do this you need to again buddy up with a 6th form/college to go in and do the experiments with their pupils.
All the best*