In the United Kingdom, a political surgery is a series of one-to-one meetings that a member of parliament may have with his or her constituents. A typical constituency has in the order of 75k residents. Are there any statistics on the contact rate of such surgeries? What fraction of the population has ever visited such a surgery? Is it a small active group that visits surgeries frequently, or are there rather many people that occasionally reach out to their MP and visit the surgery?
As an example, consider the MP for Croydon central.
She holds weekly surgeries, for two hours, 20 minutes per consultation. So she can see a maximum of 6 people per week, 300 people per year.
It is likely that not all the surgery slots are fully booked, so you may want to estimate a lower value than this. Moreover, some people will be "return customers". So let us estimate 150 individuals per MP per year.
Croydon central is a fairly typical outer London constituency. Not all MPs continue to hold surgeries. But extrapolating from this single data point one might expect 10000--100000 people to visit an MP's surgery in the course of a year, or about 0.1% of the UK population.