DramaDaisy:
Dyslexia if diagnosed should result in your DD getting extra time for reading and sometimes premission to prepare answers on computer with spell check (although I'm not clear if any of the GCSEs invovle extended essay answers).
If you haven't arranged or discussed such possibilities I'd talk to school SENCo.
Second, it's a mock - not the real thing. It's an opportunity for your DD to understand how she might do on the real thing. (with all the caveats that getting an A now doesn't guarantee an A on the real thing nor does getting a D now guarantee a D on the real thing).
A mock is quite simply a chance to take stock. Having worked hard all year - how are we doing now - a few months out from the real thing. She may find out she's on target for most subjects but a little weak in one - so can structure her time revising in the next two months such that the weak area gets a bit of extra care and attention.
Third - EXAM STRESS - can often be group hysteria. Everybody expects to be stressed, and girls go one endlessly about how stressful it is etc...., and they basically wind each other up. 'Oh my gosh - I just have to get an A or I'll die' kind of thing. Mum says if I don't get a C, I'm grounded'....etc.... girls seem particularly good at this. My 10 year old DD2 has just managed to compeltely freak herself out over a music exam because one of her friends said she was nervous about the exam and her teacher said she needed to pass to be eligible for an audition in a city orchestra. Then someone helpful pointed out Friday the 13th is bad luck. There was no need to get so wound up - it just made her tight when she played (which never helps) and made the day something of a miserable experience for her. She left in floods of tears.
Would the solution be to use the dyslexia as a 'mental' trick - these tests aren't designed for dyslexics. She's probably going to find bits of it tough going as a result. But if she adopts the attitude 'Oh well - I'll just have to do the best I can....' and treats it experimentally (won't it be interesting to see how I'm doing) - maybe that will take the stress out of the equation.
I'm not certain how your child is doing at school, but I suspect there will be subjects she is good at. I think the solution may be to remind her that in sixth form she can narrow down her course work to her strengths. So sit the GCSEs, try your best, but know that it isn't the end of the world if one or two don't go very well.
That may help to take the pressure off.