There are basically two choices: You close the borders completely - or you do what is being done and keep them open while the numbers are low, and impose restrictions as so as cases start to rise. It is inconvenient, and the travel companies in the UK are complaining bitterly - but pretending the virus isn't a threat anymore is not an option.
People should be aware of this when they choose to travel. I have some vacation time due at the end of next month, and am very reluctant to go anywhere knowing this might happen. My employer has a blanket policy of "If you choose to travel , and quarantine restrictions are imposed, that time needs to be taken off as unpaid, or worked back within 3 months"
It's a strong disincentive not to go abroad.