If somebody has Covid but doesn't know it and has no symptoms, and turns up at LHR, then the UK ideally wants them out of the country asap. A PCR test will normally take 24 hours to return a result using an on-site in-person commercial company. If that passenger can show a ticket out within 12 hours of arrival (and has suitable paperwork to enter the next country)... perhaps better to let them get onto their flight out, keep their time in the UK to a minimum, and make provision of healthcare another country's problem