You can’t raise an official dispute with transaxe anymore until day 9 after it is shown as sent.
If it’s tracked you can certainly ask the vendor well before that to check its status.
If it’s straight RM let the vendor know after 7 days and see what they suggest.
Good comms and patience resolve virtually everything 👍

Depends on what postage method your using to be honest mate.

NDD, make your vendor aware of no delivery and request your tracking to be checked for you.

1st Class - This can take anywhere from 1 - 5 days depending on Royal Mail at the time. Make your vendor aware after a couple of days that your order hasn't arrived yet. They can't track it but its best they know anyway!

Check Royal Mail status updates etc and also read a vendors profile in regards to late orders and wait times.

Hope this helps :)