DS is doing his actual GCSEs now - last one tomorrow. He was Y6 when lockdown happened, so also didn't take his SATs. Mocks are important for getting used to the austerity and formalities of exam conditions and familiarising with exam Qs, the topics which always come up, gaps in knowledge, learning to deal with the stress, the nerves, the environment. DS did 3 sets - end of Y10, before Christmas of Y11 and before Easter of Y11. By the time he got to the actual exams at the start of May, well it was pretty much business as usual. They had 3 weeks of exams before half term, and three weeks after. They were still in lessons, out of exam times, until half term.
Your dc is only half way through the courses @TheOpalFox. These Y10 mocks are as much about process and practice as anything else. DS's school allowed very anxious children to have their exams in a smaller room than the big gym. Y11 is all about helping the students learn to pass exams. Good luck to you both. Support her as much as you can - new pen, water in an unmarked bottle, clear pencil case, approved calculator, reminders to go to sleep! Get up! Have a break! Don't let her burn out.
What mocks can't emulate is the sustained 5-6 week GCSE period, but in a way real GCSEs are a bit easier, because the exams are more spread out. DS has only had 2 days with more than 1 exam and no clashes.
edit: I still don’t think that 5-6 weeks of largely written exams, with very little coursework, is the right way to assess 12 years of formal education though, for people who are still emotionally immature.