To determine the best estimate for the number of COVID-19 cases in the UK on day 22 using the graph, you would need to check the point that corresponds to "day 22" on the x-axis and see the value on the y-axis.
Since I do not have the visual graph, I recommend you look for the point on the graph corresponding to day 22 and see where it intersects the y-axis to find the total number of cases at that point.
If you have to choose from the options provided and you have observed the graph, you should pick the value that is closest to the point on the y-axis at day 22.
If you have any more specific data or can describe what the graph looks like around day 22, I can help you analyze it further!