Could you go to the police with the cyberbullying stuff?
You could at least ring 101 and speak to somebody at the police station to get advice given the school are being so useless.
Maybe being spoken to by the police will make them think twice about their behaviour... And if it screws up their gsce year - then so what - they are happy to screw your dd's year up. For cyber bullying, are you allowed to get a restraining order so that the people that have done it aren't allowed within a certain distance of your dd? If you could that would be an answer to your problems - as they wouldn't then be able to go to school if she was there, so your dd would be free of them as a result of their own actions.
Could your dd carry something that she could use to record the bullying on? Don't know what the school's policy on smartphones at school is - but maybe you record them outside of school if you see them too? Then it won't be her word against their's - there will be documented evidence. I know that recording people when they don't know is a whole different kettle of fish - but if your dd is under verbal attack from these people then it's not the same as just randomly recording people.
I suspect that if you go to the parents house that you could get into trouble or it could backfire. I'd just go down the police route and let them sort it out.
I'd also separately send a letter to the head teacher, and cc it to the deputy (if they have been involved), the chairman of the governors and the relevant person at the council. In it I'd say what has been going on, the fact that despite asking for this to be resolved it hasn't been and your dd is still being bullied at school and online. And that as such, they are failing to provide a safe learning environment for your dd as she is entitled to. So, you want to have an action plan to ensure that they do provide a safe learning environment for her - highlight that it is her GCSE year so this is a dreadful thing to happen as it could negatively impact the rest of her life, so it really needs to be sorted out quickly. Stick a timescale in and see what happens - and keep following it up and chasing.
It must be horrible for your dd not to be able to do the normal things that others do at school and to be dealing with all this on top of other stuff so just keep going back to school and detail them with every little thing. Make sure your dd keeps a log of everything that happens in school too, to be used as evidence of what is happening to her.
Hope it all gets sorted - good luck.