Wow congrats to kilash's ds. What a great recognition for him 
Dd has both proper mocks and progress tests. She had a mock in January for further maths AS and she has progress tests next week for her 3 A level subjects.
In May the two proper FM exams are in the same week as her summer exams for her other subjects. They are full papers in the hall under full exam conditions. She's worried that she won't be able to give the mocks her full effort because she'll be focused on FM.
I think if they fall on the same day she might ask if she can sit the mocks the following week. Even if just the maths teacher agreed to move hers it would lighten the load. And surely they would want her to focus on external exams?
The way universities operate these days though I wonder if y12 summer exams are the most important ones they will sit? 
Loads of kids I know got unconditionals or got in after missing their offers but the y12 exams determine what offers they get.