The short answer OP is that we could have an effective border control and quarantine policy.
Some pretend that proper border control means walling ourselves off from the world but of course it doesn't.
The problem is largely that our government is full of negative naysayers who wouldn't know a proactive policy if it not them in the behind.
It's always "We can't" and being belatedly reactive long after they've already created the next problem.
So instead we have the pointless and performative Red List pantomime which looks as if they are doing something useful, but actually is just useless bullshit.
We know this because India told the world about B.1.617.2 the Delta variant in January,
It arrived here in March,
The government were aware of this on April 1st.
But flights from India arrived every day bringing approximately 20,000 passengers from India in about three weeks.
Then on April 23rd India was finally put on the Red List.
And hundreds of flights from India have arrived since then.
The UK's border system doesn't stop variants arriving, or manage them when they do arrive.
Test, Trace and Isolate is another useless pantomime.
That's why we're in the third wave now.