I have flown into LHR twice recently, once simply to take a covid test and then leave within 48 hours, and once to stay for several weeks, including a 14-day quarantine. There is no question in my mind the proposed itinerary would work. The passenger will need to complete the Passenger Locator Form within 48 hours of arrival at LHR, but clearing immigration will not be a problem (unless eligible to use the e-gates, the passenger should be prepared to show confirmation of completion of the Locator Form on their phone), and then the passenger can make their way to Eurostar by taxi or public transit (or walking, or whatever) without any problem.