Far too simplistic and easy to say 8 months later.
In fairness it was said (and done by some countries) at the time.
How long should the borders have been closed for?
Indefinitely.
Are we just talking about flights, or all borders?
All borders to non-essential movement of people.
Should the borders have been closed for everyone or just non nationals?
Nationals told to return. Non-nations have to apply for a visa with very limited reasons for granting. Mandatory testing and isolation on arrival for all. So closed really just means restricted.
Closing borders would have just disproportionately effected different sectors: food and fuel prices ect would have increased.
Why? No one is suggesting limiting trade.
The main negative would be tourism but that would be more than balanced by Brits staying here. Universities would have lost income.
It's far from ideal given the nature of our economy but countries which have done this for a year managed OK. If we'd done it early enough it would certainly have been a nett positive.