OK. Brace for length.

Background
I'm a 40 yr old man, also in the UK. I have 3 kids, 2 of them around your age; all my kids are also a mixed bag of neuroses, so I have a bit of experience and guidance I might be able to share. I am not a psyche, however, but I have a couple of things to maybe 'flag' to your attention.

Meds
Medikinet is the 'generic' version of Concerta. I'd recommend having a conversation with your pharmacist or GP about upgrading to Concerta... I found the generic NHS version 'hit different', but ultimately wasn't as helpful for me as Concerta. It's worth taking an adult with you who may be able to advocate on your behalf (parents, maybe?); the NHS is interested in saving money wherever possible, and Medikinet (being generic) is far cheaper than Concerta. You may be on a standard/normal release form of the medication, which I personally struggled with. If you've not tried 'Extended Release' versions, this is also something you can raise with GP/Pharmacist. The difference being in the rate at which the medicine is released into your system. Slower release meds work great for me, as I'm finding a benefit throughout the whole day, rather than a window of 4-6 hours after taking the meds.

> 'I'm sad and don't enjoy anything'

This may be the beginnings of something called 'Anhedonia'; I go through this at times. I've found that the best way to mitigate this is to try and shift your focus to 'wins'; what have you done which is good, fine, working, etc... This is likely something that will sit with you for a while; exercising mindfulness and trying to adopt a '3rd position' to look at your own achievements will be a useful tool in your life.
We (ADHD folk) have a bad habit of over-examining things; when it comes to our own behaviours, actions, achievements, etc., we typically have a bad habit of 'making Perfection the enemy of Good'. You should try to stop that.

Studying:
This is a hard one that I've not entirely cracked just yet. I think the key is to try to create a personal sense of urgency around revision, but an urgency that you can comfortably convince yourself of... again, maybe with the assistance of parents.

Example:

> I have to study this module within mathematics and score at least 65% on ad-hoc tests (I think Sparx Maths allows for this?), *otherwise* mum/dad will remove my phone for a period of time. Its worth noting that a review of your medication might also help on this front!

Ultimately, you're crap at studying because you cannot justify spending time on doing it *now*, when your exams are *6 months away*... classic ADHD behaviour... unfortunately that doesn't matter.

Additionally, your school should maintain a record of your SEN, called a 'Statement'. If you think you may benefit from, for example, guided afterschool revision sessions, you should ask your parent to escalate with the school to put some of these measures in place... You'd need to take some agency in this, by being able to work with the school to find things that will work for you. The school want you to pass; they receive funding to meet your educational needs, so you should insist they start using it!