Actually the IAAF posted the Provisional Entry Lists, which also includes reserves, providing they have the Qualifying Standard. The Final Entries are submitted about 48 hours prior to first round, and it is at this point that the National Federation must declare with 3 of the 4 entered.
If you were to look at some other events, you would see for example that Germany has 5 Javelin Throwers entered, which includes their Defending World Champ Wild Card entry. This allows you to substitute if one athlete were to get injured in the final days. If they were not on the initial entry list, then it would be too late to add another athlete, and you would not look at IAAF Invitations as this was too late. It is just the nominated reserve that you could substitute.
Now we just need for UKA to submit the correct names this time, and not the reserve shot putter that they entered for the 4x400m Relay at European Team Champs!