@Debroos: Cases are recorded at the location they were reported. Thus, those who reported positive (e.g. via NHS Track 'n Trace) gave rise to a case in the region they were at the time they reported positive. Thus, holidaymakers who reported positive while in Cornwall got added to the Cornish total. However, the population figure used to calculate the rate is that of the permanent population, and does not include tourists. Here's a worked example:
Let's consider a region with a permanent population of 10,000 that during the height of the season swells to 40,000. Now assume that in a week at the height of the season, 40 people report positive.
The official Covid rate is calculated as (40/10,000) * 100,000 = 400 per 100,000.
However, the positive cases were reported when the actual population (including tourists) was the seasonal high of 40,000, not the 10,000 used in the official calculation.
Hence the actual Covid rate is (40/40,000) * 100,000 = 100 per 100,000.
Of course, you have to use the actual figures for whichever area is under consideration.