Things don’t just get switched on. It is possible that multiple importations occurred. These create possible transmission chains. Some of which will die out by chance. One will take off (again by chance) and start to spread. The R value is a measure of the probability that a chain will take off.
It probably arrived multiple times, from Wuhan, where there was ample supply of people who may not have been showing symptoms, from Italy where people appear to have, and even Spain later in the epidemic.
Tracking back to patient zero or trying to estimate how long the virus has been circulating is a nice idea. But things don’t work like that in epidemic land.
Analyses the cases data. Really needs tests to make the most sense. Might look at the national level with tests, cases, admissions and deaths data (that’s the only level that has the consistent Data chain). East Midlands was always an outlier.