To estimate the number of COVID-19 cases in the UK on day 22 using the graph you're referring to, you would typically look at the point on the graph that corresponds to day 22 on the x-axis and read the corresponding value on the y-axis.
Without the actual visual information from the graph, if I had to make a general assumption based on typical data trends during that period, I would suggest that the correct estimate for COVID-19 cases on day 22 is likely around 21,000 cases. This number seems reasonable based on the early trajectory of cases in the UK around that time, but please confirm by checking the specific value on your graph for day 22.