I'm not in your area but when my youngest dd did her 11+ exam she did sit quite a few. At first I didn't really believe in them but they really did iron out the nerves and highlight silly mistakes.
You would think a child would fill in a multiple choice sheet correctly but it's the pressure of being in a strange environment, using the old brain for 2-3 hours
, the noisy sounds. In one of the sutton mocks my daughter got below 50 for english which was shocking- but she told me she made an error with the answer sheet which had a compound effect on her score. But because she went through this and learnt from her mistake she got a better score in the second round.
If you don't want to go for the mock make your child practise in noise or with family so they can learn to tune things out.