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.
A friend of mine is travelling from Cairo to Paris today and then on to London. Considering Egypt is a red light zone, France is green the whole system seems absurd!! Additionlly Egyptair restart their LHR schedule today, so on arrival at Terminal 2, what happens when passengers mix with flights from green light zones at immigration? Am I missing something here?