Midweek term time would be very quiet yes, but to be honest, I've always found it quiet there compared to other parks. We've never had to queue for anything longer than 1/2 an hour, even on a Saturday in the summer holidays. The troublesome trucks had the biggest queue, I think.
I've found the email, reprinting here, hoping you find it useful:
"Thank you for your enquiry. I must admit it is very valid information that you require, however, we have never had an enquiry of such before which is probably why the issue is not addressed in any of our literature. As previously stated the access bands are there to help those who find queuing in a busy area difficult (the exit area allows a maximum of 5 guests to wait at any one time so there are fewer people around), however the theme park itself has not had information printed on areas to recover from sensory overload.
The medical centre is a fairly large indoor area that is usually quiet and available to guests throughout the day if they do require some time away from the park. However, use of this area may be restricted if emergency aid to a guest within the park is necessary.
There are also picnic areas around the park that tend to have less people around, (particularly one hidden away just before entering the zoo on the left hand side and an undercover area next to the Excalibur attraction) however, announcements and background music may still be heard in these areas and the amount of people in these areas often depends on if the park is busy or not.
The best area I can advice to use for a closed off contained area would be a room next to the zoo toilets. If you telephone in advance we can ensure this room is available for members of your party to use when necessary on the date of your visit. In order to arrange this please contact us on 01827 252400 and ask to be put through to the first aid department."