On today, the longest day of the year, a list of London’s longest:

Longest borough – Hillingdon (12 miles)
Longest postcode – CR0 (7.5 miles)
Longest dimension – M25 J14 to North Ockendon (36.5 miles)
Longest river – Thames (44 miles)
Longest canal – Grand Union (16 miles)
Longest flight of locks – 7 (Hanwell)
Longest canal tunnel – Islington (878m)
Longest bridge – Waterloo Bridge (370m)
Longest island – Brentford Ait (620m)
Longest reservoir – King George’s Reservoir (4 miles)
Longest runway – Heathrow 09L/27R (2.4 miles)
Longest flight – Heathrow to Perth (9009 miles)
Longest footpath – London Outer Orbital Path (150 miles)
Longest park – Richmond Park (2.7 miles)
Longest street – Green Lanes (7.5 miles)
Longest straight road – the A5 (Edgware Road) (10 miles)
Longest motorway – M25 (approx 117 miles)
Longest mews – Pavilion Road, SW1 (900m)
Longest streetname – Alfred’s Way (East Ham and Barking By-pass)
Longest ‘street’ name – St Martin-in-the-Fields Church Path
Longest one word streetname – Straightsmouth
Longest Underground line – Central line (West Ruislip → Epping, 34.1 miles)
Longest Underground journey – Uxbridge → Cockfosters (32 miles)
Longest Night Tube journey – Heathrow T5 → Cockfosters (29 miles)
Longest non-stop tube journey – Finchley Road → Harrow-on-the-Hill (7.2 miles)
Longest non-stop rail journey – Paddington → Heathrow (14.5 miles)
Longest tunnel – Thames Water Ring Main (50 miles)
Longest Underground tunnel – East Finchley → Morden (via Bank) (17.3 miles)
Longest rail tunnel – Stratford → Dagenham (6.5 miles)
Longest road tunnel – Limehouse Link (1.1 miles)
Longest foot tunnel – Woolwich (504m)
Longest station name – Caledonian Road and Barnsbury
Longest tube station name – High Street Kensington
Longest non-TfL station name – West Hampstead Thameslink
Longest one-word station names – Knightsbridge/Woodmansterne
Longest tube escalator – Angel (61m)
Longest escalator – Heathrow Terminal 5
Longest bus route – X26 (Heathrow → Croydon, 24.1 miles)
Longest nightbus route – N199 (St Mary Cray → Charing Cross, 22.1 miles)
Longest bus stop name – Loxford School Of Science and Technology
Longest-running play – The Mousetrap (since 6 October 1952)
Longest-running musical – Les Misérables (since 28 September 1985)
Longest market charter – Barking (since 1175)
Longest-serving MP – Harriet Harman (since 28 October 1982)
Longest-serving male MP – Jeremy Corbyn (since 9 June 1983)
Longest drought – 73 days (Mile End, spring 1893)
Longest period of continuous rain – 59 hours (13-15 June 1903)
All data courtesy from Diamond Geezer who assumes several of these are wrong; and if they’re wrong factually than pedantically.