Quarantine hotels will have to cover all arrivals otherwise it's just pointless. If only a small number of countries are on the list then people will just return to the UK via a 3rd country to avoid the requirement to quarantine. Which is exactly what happened with the 'Dublin Dodge' when the requirements to quarantine at home were introduced.
I'm sorry to say that if the plan is to prevent a yet undetected mutant strain entering the UK, quarantine hotels will run for most of 2021. And in all fairness, this should have happened a year ago.
There is not going to be a Summer 2021 holiday season in any meaningful sense.