It's generally accepted that imposing travel restrictions on their own is more or less pointless unless you do it pre-emptively, because the motivation to close the borders is usually after the disease has arrived rather than before. On the other hand, maintaining open borders and waiting for the disease to spread freely before doing anything at all beyond just watching has had devastating and tragic consequences.
The real problem is that the UK didn't have the ability at the start to do much at all to track the disease, which means that in February/March the only options were drastic. Lockdown is a very blunt instrument -- ditto closing the borders, and even the idea of mass quarantines is tricky. In that sense I agree with NJ that at the start not much more could have been done. On the other hand, that's still a failing, and hopefully the UK* will learn in future to be able to take swift action in order to stop or severely reduce the spread of any future pandemics, which will include being able to set up border health screenings or the like.
*This is not a UK-specific failing, I hasten to add -- but we could still do with learning it.