I can't see any logic to not having a quarantine because it's easier to move about within the UK. It's like saying that by pouring petrol onto a house fire will yield the same outcome of a house being on fire, however by not pouring petrol onto the situation gives a better chance of trying to put the fire out. It just makes sense by not adding more fuel to the fire by quarantining people returning from foreign travel.
Air travel has been identified as a particular hotspot for transmission, and
Cardiff had a great example last month. Movement in the UK is not really done through an air pressurised cabin, so it's comparing apples with oranges.
Agree that it's a shambles. We should be aware that to combat this virus prior to any vaccine, it'll need a multiple combinations of smaller actions to get it under control, quarantining being one of them.