I don't understand why the UK borders weren't closed to ALL travelers, back in March.
Because:-
a) Many of the people still travelling were people returning 'home'. Closing your borders and preventing your own citizens (and people who have made their home here) repatriating themselves would have been unthinkable.
As it was, there were people who had
chosen to go and live in a different country who were bitterly complaining that the UK Government wasn't providing repatriation flights
free of charge. I don't remember very many media headlines supporting the Government's stance, and cannot imagine any of them would have supported the Government in saying 'Keep out'.
b) The virus was already widespread in the UK population. It was already far too late to close the borders to keep the virus out. At that point, having a handful of additional cases arriving in the UK from elsewhere makes no difference - especially if they are people arriving from a place where the infection rate is lower.
I don't understand why emergency legislation wasn't passed to make it an offence not to follow isolation rules.
And followed by arrests, fines and, when necessary, goal terms.
It was.
But the issue was, and will always be, how do you enforce the rules without making the situation worse?
There were people demanding that prisoners be released to prevent them all dying in prison - in that situation how do you go about rounding up rulebreakers and throwing them in gaol? What happens when the first elderly person you've flung in gaol for breaking the rules then dies from CV-19?
Hardline enforcement of the rules would have lead to protest and possibly riot - the very opposite outcome to the one you wanted.
ALL of the above happened on the Isle of Man 
Result: the island has eradicated Covid-19.
The Isle of Man is very fortunate in terms of geography, demographics and economics. Ditto some other places such as the Channel Islands.
There are many people in mainland UK who dearly wish we were more like you in that regard.
In mainland UK these people are typically called Xenophobes, Racists, Bigots, Gammons, Thick, etc etc.
And what worked for the Isle of Man wouldn't have worked for mainland UK.