So, looks like the new policy will be a "traffic light" system, with each country being assigned a colour code which will be reviewed regularly. Green - safer than the UK, amber - less safe than the UK, red - self-isolation required upon arrival. Nothing is mentioned in the way of visits to countries preceding the one from which the entry into the UK is made. For example, if you are coming from a green country but 3 days ago you visited a red one, that doesn't seem to make any difference. Which will likely give the upper leg in terms of connecting flights to the UK to the green countries.