The most sensible form of quarantine, if there should even be one, would be for HMG to say that the 14day is a blanket for any country that has an R0 higher than the UK. That would be saying that any country that is currently at higher risk of spreading than us would have it's arrivals quarantined.
This would give them the ability to move the goalposts as appropriate, although that doesn't help in terms of any certainty going forward.