BBC iPlayer is the BBC's catch-up and live streaming service, offering everything from Doctor Who and Peaky Blinders to live BBC One coverage. It's funded by the UK TV licence fee, which is why it's geofenced to UK viewers only — outside the UK, iPlayer returns a 'BBC iPlayer only works in the UK' message even if you have a UK TV licence and a UK BBC account.
A VPN with a UK server bypasses this geofence by giving you a UK IP address. Combined with a free BBC account (which only requires a UK postcode — any valid one works), you get the full iPlayer catalogue including live BBC One, BBC Two, BBC News, and on-demand archives.