Judging by the report in the "I" this morning delays at the UK border could be horrendous, due in part because the UK government has yet to get vaccination certificates into an online / app system.
There is part of me that thinks the government feels duty bound to open up international travel from 17th May, but then doesn't want to do anything that will make the whole process of leaving and returning to the UK simple or hassle free to put people off travelling in the first place.