Usually - as in London - tour guides are expected to know and understand the history or background of a particular locality, tourist attraction or even a whole city. There is a Board with examiners who test the extent of their knowledge. The City of London, known as the Square Mile has several well-known sites such as the Bank of England, the Tower of London, the Guildhall and various archaeological sites that are currently open to the public, the City of Westminster has Westminster Abbey, the Houses of Parliament, Buckingham Palace, several large museums and galleries, decorative or architecturally acclaimed stations and of course there's Harrods! In Southwark and Lambeth there is the Imperial war Museum, South Bank Centre, a complex of theatres, a cinema and concert venues, Southwark Cathedral and the Tate Modern as well as the 'Oxo Tower', and a river-front collection of restaurants and exhibitions. Greenwich has the Naval College and hospital that is now a gallery and conference centre, the Queen's House and Greenwich Park with its Royal Observatory on the Meridian line... These are things you would have to know about, inside out, as a guide, dates and all.
There are also sports venues such as Wembley Football Ground, Arsenal and others that have their own guides, and Lord's Cricket Ground in NW8. The tour at Lord's is about an hour and forty minutes long and guides must know a good deal about the game (as at the football grounds) as well as the history of the grounds (yes, there were more than the one in current use) and the founder Thomas Lord of Thirsk in North Yorkshire. There is a routine that can be changed depending on circumstances, and on major match days there are no tours and tour guides double as stewards who need to know what is where... And they learn the hard way if they get caught out passing on 'duff' information by knowledgeable 'punters' such as Middlesex Cricket Club (MCC) members who 'breathe' cricket!
Still want to be a guide? By the way wannabe tour guides at Lord's follow a seasoned guide a few times, then go on a 'dry run' before being fed to the lions.