Are there no checks at UK airports, no records kept to enable monitoring of individual passengers & their contacts if it later proves necessary ? 
Surely must be some ?
e.g. This is what Germany has been doing:
https://www.bundesgesundheitsministerium.de/en/press/2020/coronavirus.html#c17363
"Before landing in Germany, pilots on flights from Iran, Italy, Japan, South Korea or China must transmit the Aircraft Declaration of Health to the authorities.
Before disembarking from the aircraft or ship, passengers arriving in Germany from Iran, Italy, Japan or South Korea by plane or ship,
must provide information on their flight/trip and on where they can be reached for the 30 days after arrival.
This is done with the disembarkation card:
Passengers from China to Germany must provide additional information on where they were staying in China, persons with whom they were in contact and their state of health.
In this case, this disembarkation card must be used.
Airlines, ship-owners, charterers and all other persons and all other persons responsible for operating ships must keep the data on flights and trips to Germany from Iran, Italy, Japan, South Korea and China available for a period of 30 days after arrival."
There are also measures for those arriving by train whose original departure point came from one of those countries