Yes, you are allowed to wear the pump. Mock exams are internal school run exams and don't involve an external exám board. If the school makes you remove your medical device for that, they would be seriously breaching their Equality Act duties.
Even when the exams are external exam board exams they are still subject to the equality act and you will be allowed your pump.
You should make sure to have a discussion with your school about it in advance of the exams so they are aware of your pump and let the invigilators know so they don't think you are smuggling a calculator up your jumper.
You should probably check out the Jdrf guidance type 1s on exams -
https://jdrf.org.uk/living-with-type-1/everyday-life/school-and-university/exams/