They seem to have added a section to the info on the tui website now that also states if you need to claim a refund sooner (before the credit voucher is issued) you can call to do so.
If you’re unable to accept a refund credit you can apply for a refund; due to the volume of holidays impacted, we’re asking you not to contact us until you receive your refund credit so we can continue helping customers in date order. However, if you do need to request your refund before this you can do so using the details below. Please be aware that our call centre teams are incredibly busy, so call waiting times are considerably longer than usual. You can call 0203 451 2868 between 9am and 7pm from Monday to Friday.