I'm not sure how much of an 'issue' it is. If a GAR is required it has to be done.
https://www.gov.uk/government/public...ion-of-flights
Details here. Including an online form Sanhor.
If it's not done and they happen to be waiting for you expect to have an issue. But the requirements are laid out clearly so there's no real excuse. And abusing the system only ruins it for everyone.
In my dealings I've always found them efficient and courteous. Any changes are accepted with no problems. One time the pax had departed then BF arrived late. They were a bit shirty till I pointed out that they hadn't told anyone they were coming and I was on time. Problem over. Always carry a copy.