I think the question of not letting UK citizens in for Summer '21 will differ depending on the destination. For example, I think the Canaries will open up as soon as they can with a thorough testing program as mitigation. I don't think the Spanish government nor Brussels will be throwing sufficient support money at autonomous regions or EU countries who heavily depend on tourism. And for those places, there's no point having a lockdown if your citizens end up on the streets rioting against poverty, and also damaging your reputation as a tourist hub.
The key bit is all countries getting those at risk of death or serious illness vaccinated, repeated at regular intervals as necessary. Once the death rate and hospitalisation rate drops, the number of cases / contagiousness becomes irrelevant.