There’s lots of good advice here, but you need to be a bit careful. Telling one teacher doesn’t mean it will get passed on to everyone who is invigilating. Also lots of schools use invigilators who they employ just for exams, so you may well be in an exam with no-one that you recognise invigilating.
Most schools have an Exams Officer. This might be a teacher, or it might be one of the Admin staff. If you ask your form tutor they will know who it is. If you talk to the exam officer, or let your form tutor do it on your behalf then that should mean that the information will get to everyone who needs to know. The exam officer will also know if you are likely to be allowed extra time. This has to be applied for well in advance from each exam board, and s/he will be able to do that (you can’t do it yourself).
The sooner you talk to the exams officer the better, because the process can take some time as the exam boards often come back with a request for more information.
I’m an ex head of 6th form, and our exams officer spent a lot of time making sure that everyone got what they were entitled to. He liked to get everything sorted out well in advance. There were only last minute emergencies like students turning up with their writing arm in plaster, so there would be a last minute panic to find someone to write the exam for them with the answers being dictated.
I hope you do well in the mocks, and in the summer exams.
Margaret