Whilst the Age UK shop in Dover supports our local centre exclusively, it is true that many charity shops profits go to a regional or national body.
It is also true that direct contributions/membership subscriptions, legacies etc. go to the central body for use not only on their chosen good cause but the costs of running a regional or national organisation. These cost vary widely from charity to charity and must be put in the context of what the charity does and how it delivers its charitable remit.
Having said that charities must be transparent about their cost base
https://www.theguardian.com/voluntary-sector-network/2016/apr/26/charities-admin-costs-accounts-reporting
Finally a website where you can compare what charities spend on running costs etc -
http://www.aliveandgiving.com/