I did my first non UK flight in 20 months on Monday to LIN back wednesday. BA app wouldn't let me complete the uploads as I didn't have a negative covid test. I got this done on the morning I flew at T5 so it would be valid for my return trip. There were delays at LHR for this so I only got the result 5 minutes before check in closed so did a manual check in. I then tried to upload everything for the return but the form was stuck from Monday with Monday's date and flight and I couldn't complete it either.
I had the email about Verifly but realised when I tried to use it Milan isn't a Verifly destination. I was then confused by the Verifly check in lane at LIN, but reading this thread I now know that it can be used for flights back to the UK! I also couldn't get the UK PLF to accept my NHS covid vaccination certificate, said the file size was too small?. A message came up saying it would need to be checked at the airport, but wasn't.
All I can say is that travel isn't what it was. I have never had to compete so many online forms (or fail to complete so many online forms) to then have to complete similar forms on paper on board the plane or at the airport again. Most of which are either shoved in a folder or not wanted.
I'm glad I printed everything out and just had a folder with it all in, something I haven't done in many years but times are going backwards I fear.
Advice give yourself and extra hour at the airport and have everything printed as well as electronically. Be prepared for inconsistencies and ideocracy.