If you receive the message "Shipment booked OK but we can not update tracking number to Royal Mail in TikTok. Tracking number mismatch provider". The chances are that the shipment has been booked with a "Non-Tracked" Royal Mail service.
For Royal Mail Tracking to pass back to TikTok, the shipment needs to be booked with a "Tracked" Royal Mail service such as "Royal Mail Tracked 48 No Signature".
Tracking numbers are 13 digits: 2 letters + 9 numbers + "GB"
"Only Royal Mail Tracked 24/48 is acceptable; 1st/2nd class and RM 24/48 are not trackable services."